ID: Viral, fungal and parasitic infections

• Viruses: Herpes Simplex | Chickenpox | Shingles | Epstein–Barr Virus | Cytomegalovirus | Influenza | COVID-19, RSV and Other Respiratory Viruses | Measles, Mumps and Rubella | Parvovirus, HHV-6 and Enteroviruses | Mpox | Fever and Rash — Viral Causes at a Glance
• Fungi: Candida Infections | Aspergillus | Mucormycosis, Pneumocystis and Endemic Fungi
• Parasites and prions: Toxoplasmosis | Giardia, Cryptosporidium and Amoebiasis | Worms | Strongyloides | Prion Disease
• Revision: Drug Reference Table | Spot the Diagnosis | Exam-style Scenarios | Key Points | Guidelines and Further Reading

🦠 Viral, fungal and parasitic infections — page 4 of the Infectious Diseases chapter. Most viral infections are self-limiting, but some are dangerous in pregnancy (chickenpox, rubella, CMV, parvovirus, toxoplasma, measles), in the immunosuppressed (herpes viruses, fungi, Strongyloides), or spread rapidly in hospitals (measles, chickenpox, influenza). Fungal infections are rare in healthy people but common and deadly in neutropenia and critical illness. HIV and hepatitis viruses are on page 3; malaria and tropical parasites on page 5; skin infections and childhood rashes have their own chapters.
Your four questions for every patient: Is the patient pregnant, immunosuppressed, a newborn, or otherwise at high risk? Is antiviral, antifungal or antiparasitic treatment needed, and how soon? Do they need isolation (airborne or droplet)? Do contacts — especially pregnant or immunosuppressed people — need protection?

Herpes Simplex

• HSV-1 — mostly mouth and face (cold sores), encephalitis, and increasingly genital herpes; HSV-2 — mostly genital herpes and recurrent viral meningitis. Both become latent in nerve ganglia and reactivate.
• Forms:
  • Primary gingivostomatitis (children) — fever, painful mouth ulcers, drooling, refusing to drink (watch hydration).
  • Cold sores — tingling, then vesicles on the lip; triggered by sun, fever, stress.
  • Herpetic whitlow — painful vesicles on a finger (don't incise).
  • Eczema herpeticum — widespread punched-out painful erosions on eczema — emergency: IV or oral aciclovir, same-day review (especially near the eyes).
  • Herpes keratitis — dendritic corneal ulcer (fluorescein); red painful eye; urgent ophthalmology; no topical steroids.
  • Genital herpes — see the Sexual Health chapter.
  • Neonatal herpes — from maternal genital infection at delivery (highest risk with a primary infection in the last 6 weeks of pregnancy — caesarean section advised); disseminated disease, encephalitis — high mortality — IV aciclovir.
  • HSV encephalitis — see page 2.
• Diagnosis — swab of a vesicle for HSV PCR.
• Treatment — oral aciclovir or valaciclovir for primary infections, severe recurrences, eczema herpeticum and immunosuppressed patients; topical aciclovir for cold sores helps only a little if started early.

Chickenpox

• Varicella-zoster virus (VZV) — primary infection is chickenpox; it then lies latent in sensory ganglia and can reactivate as shingles.
• Spread — airborne and contact; incubation 10–21 days; infectious from about 2 days before the rash until all lesions have crusted (usually about 5 days after the rash starts).
• Features — fever, then an itchy rash in crops (spots → vesicles → crusts) starting on the face and trunk, with lesions at different stages at once; mouth lesions.
• Complications (more common in adults, pregnancy, smokers, the immunosuppressed and newborns):
  • Secondary bacterial infection — including invasive group A strep and necrotising fasciitis (avoid NSAIDs in chickenpox).
  • Varicella pneumonia (adults, pregnancy, smokers) — breathlessness, cough — can be life-threatening.
  • Encephalitis, cerebellar ataxia (children, usually recovers), hepatitis, haemorrhagic and disseminated chickenpox (immunosuppression).
• Treatment — supportive for healthy children (paracetamol, fluids, emollients, antihistamines); oral aciclovir (or valaciclovir) within 24 hours of the rash for adults, adolescents, pregnant women and those with severe disease; IV aciclovir for the immunosuppressed, pneumonia, encephalitis or severe disease.
• Pregnancy:
  • Exposure — check VZV IgG (most are immune); if not immune, give oral aciclovir or valaciclovir from day 7 to day 14 after exposure (UKHSA); VZIG if antivirals are unsuitable.
  • Chickenpox in pregnancy — oral aciclovir within 24 hours of the rash; urgent assessment if chest symptoms, neurological signs, haemorrhagic rash or dense lesions.
  • Fetal varicella syndrome (infection before 20 weeks, about 1–2%) — limb, eye and brain defects, skin scarring.
  • Neonatal chickenpox — maternal rash from 7 days before to 7 days after delivery — severe; baby needs VZIG and/or aciclovir.
• Immunosuppressed contacts — check immunity; post-exposure antivirals (or VZIG) as advised.
• Prevention — varicella vaccine (live) for non-immune healthcare workers and close contacts of immunosuppressed people, and now being added to the routine UK childhood schedule (as MMRV); exclude from school or nursery until all spots have crusted.

Shingles

• Reactivation of VZV in one sensory nerve — more common with age and immunosuppression.
• Features — burning or tingling pain for days before a vesicular rash in one dermatome that doesn't cross the midline (often thoracic); fever, malaise.
• Special forms:
  • Herpes zoster ophthalmicus (V1) — vesicles on the forehead and around the eye; Hutchinson's sign (vesicles on the tip or side of the nose) predicts eye involvement — same-day ophthalmology plus antivirals.
  • Ramsay Hunt syndrome — vesicles in the ear canal or palate, facial palsy, vertigo, hearing loss — aciclovir plus prednisolone; ENT referral.
  • Disseminated zoster (more than about 3 dermatomes or widespread) — in immunosuppression; airborne precautions; IV aciclovir.
  • Motor involvement (limb weakness), zoster meningitis or encephalitis, stroke (VZV vasculopathy).
• Treatment — oral aciclovir 800 mg five times daily or valaciclovir 1 g three times daily for 7 days, ideally within 72 hours of the rash (NICE CKS) — for people over 50, immunosuppressed people, non-truncal shingles (face, eyes, limbs, perineum), moderate or severe pain or rash; any time for ophthalmic zoster or immunosuppression. Good pain relief.
• Infectiousness — non-immune people can catch chickenpox from direct contact with the fluid in the vesicles until they crust; cover the rash; avoid contact with pregnant women, newborns and immunosuppressed people.
• Post-herpetic neuralgia — pain lasting over 3 months, more common with age; treat as neuropathic pain (amitriptyline, duloxetine, gabapentin, pregabalin; capsaicin or lidocaine plasters).
• Prevention — shingles vaccine (recombinant, non-live — Shingrix, 2 doses) offered to older adults and to people with severe immunosuppression from age 50 (and some younger people) under the UK programme.

Epstein–Barr Virus

• Infectious mononucleosis (glandular fever) — mainly teenagers and young adults; spread by saliva ("kissing disease"); incubation 4–8 weeks.
• Features — sore throat with exudative tonsillitis, fever, posterior cervical lymphadenopathy, marked tiredness, palatal petechiae, splenomegaly (about half), mild hepatitis (raised ALT, sometimes jaundice), eyelid swelling.
• Amoxicillin or ampicillin given for "tonsillitis" causes a widespread itchy rash in most patients — not true penicillin allergy.
• Tests — atypical lymphocytes on the film, raised ALT; heterophile antibody (Monospot) — may be negative in the first week and in young children; EBV-specific serology (VCA IgM) if needed. Consider primary HIV if EBV tests are negative.
• Management — supportive (paracetamol, fluids); fatigue can last weeks to months; return to school or work when well (no exclusion needed).
• Complications:
  • Splenic rupture (rare — usually in the first 3 weeks) — avoid contact sports and heavy lifting for at least 4 weeks (often 8); sudden left upper quadrant or left shoulder pain → emergency.
  • Airway obstruction from huge tonsils — admit; steroids.
  • Haemolytic anaemia (cold agglutinins), low platelets, hepatitis, Guillain–Barré, encephalitis, myocarditis.
• EBV and cancer — Burkitt lymphoma, nasopharyngeal carcinoma, Hodgkin's lymphoma, post-transplant lymphoproliferative disorder, primary CNS lymphoma in HIV; oral hairy leukoplakia in HIV.

Cytomegalovirus

• Healthy people — most infections cause no symptoms; some get a glandular fever-like illness (Monospot-negative) with hepatitis; lifelong latency.
• Congenital CMV — the commonest congenital infection in the UK:
  • From primary or reactivated infection in pregnancy.
  • Effects — sensorineural hearing loss (the commonest), microcephaly, developmental delay, chorioretinitis, periventricular calcification, low birth weight, hepatosplenomegaly, low platelets.
  • Prevention — hygiene advice in pregnancy (hand washing after nappy changes and contact with young children's saliva; don't share cutlery with toddlers).
  • Symptomatic babies may be treated with valganciclovir (specialist, ideally started within the first month).
• Immunosuppressed people:
  • Transplant recipients — CMV viraemia, pneumonitis, hepatitis, colitis, graft dysfunction; managed by regular CMV PCR monitoring with pre-emptive treatment, or prophylaxis (valganciclovir, letermovir).
  • Advanced HIV (CD4 under 50) — retinitis, colitis, oesophagitis, encephalitis.
• Treatment — ganciclovir (IV) or valganciclovir (oral) — marrow suppression (FBC), kidney dosing; foscarnet or maribavir for resistance.
• Blood products — leucodepleted components reduce CMV transmission; CMV-negative components for intrauterine transfusion and neonates.

Influenza

• Influenza A and B — winter epidemics; influenza A also causes pandemics (new strains, sometimes from birds or pigs).
• Spread — droplets, contact; incubation 1–4 days; infectious for about 5 days (longer in children and the immunosuppressed).
• Features — sudden fever, chills, muscle aches, headache, dry cough, sore throat, exhaustion; older people may present with confusion or a fall.
• Complications — viral pneumonia, secondary bacterial pneumonia (including severe S. aureus necrotising pneumonia), exacerbations of COPD, asthma and heart failure, myocarditis, encephalitis, MI and stroke risk in the weeks after infection.
• At-risk groups — over 65, pregnant women, chronic heart, lung, kidney, liver or neurological disease, diabetes, immunosuppression, severe obesity, care home residents, young children.
• Diagnosis — nasopharyngeal swab PCR (often as part of a respiratory viral panel); droplet precautions in hospital (side rooms or cohorting).
• Treatment — oseltamivir (5 days) for at-risk people with influenza-like illness when flu is circulating, ideally within 48 hours of symptoms, and for all hospitalised patients with suspected influenza (even later); zanamivir for resistance or severe illness; treat bacterial complications.
• Prevention — annual vaccination (inactivated injectable vaccine for adults and at-risk groups, including all pregnant women; live nasal vaccine for children); post-exposure antiviral prophylaxis for at-risk contacts in outbreaks (e.g. care homes); staff vaccination.
• Avian influenza (e.g. H5N1) — exposure to sick or dead birds or poultry; severe pneumonia; isolate, notify, specialist advice.

COVID-19, RSV and Other Respiratory Viruses

• COVID-19 (SARS-CoV-2):
  • Ranges from no symptoms to severe viral pneumonia, ARDS, clotting (VTE, stroke), myocarditis and multi-organ failure; highest risk in older people, the immunosuppressed and those with chronic disease.
  • Early antivirals for people at high risk of severe disease (within 5 days of symptoms) — nirmatrelvir–ritonavir (major interactions), sotrovimab or remdesivir (NICE).
  • Hospitalised patients needing oxygen — dexamethasone 6 mg daily for up to 10 days; tocilizumab or baricitinib in severe disease (specialist); VTE prophylaxis; prone positioning and respiratory support.
  • Long COVID (post-COVID-19 syndrome) — fatigue, breathlessness, brain fog and other symptoms beyond 12 weeks.
  • Seasonal vaccine boosters for older and at-risk groups.
• RSV (respiratory syncytial virus):
  • Bronchiolitis in infants (see the Paediatric Respiratory chapter); pneumonia and exacerbations in older adults and the immunosuppressed.
  • Prevention — RSV vaccine for pregnant women (protects the baby) and older adults under the UK programme; monoclonal antibody prophylaxis for some high-risk infants.
• Other respiratory viruses — rhinovirus (common cold), adenovirus (conjunctivitis, pharyngitis, severe in transplant recipients), human metapneumovirus, parainfluenza (croup), seasonal coronaviruses. Treatment is supportive; don't prescribe antibiotics for uncomplicated viral infections.

Measles, Mumps and Rubella

• Diagnosis — oral fluid (saliva) swab for PCR and IgM (kits via UKHSA), or blood serology and throat swab PCR; notify on suspicion.
• Measles in hospital — airborne isolation (negative pressure room, FFP3, only immune staff); don't let a suspected case sit in a waiting room.
• Measles exposure — MMR vaccine within 3 days for susceptible contacts over 6 months; human normal immunoglobulin within 6 days for susceptible pregnant women, immunosuppressed people and infants (UKHSA).
• Prevention — MMR vaccine (live) at 1 year and 3 years 4 months (two doses); check and offer MMR to anyone who missed doses (including adults and healthcare workers); outbreaks in the UK follow falling vaccine uptake; MMR is not given in pregnancy (offer after delivery to non-immune women).

Parvovirus, HHV-6 and Enteroviruses

• Parvovirus B19:
  • Children — erythema infectiosum ("fifth disease", "slapped cheek") — bright red cheeks then a lacy rash on the limbs; infectious before the rash (no exclusion needed once the rash appears).
  • Adults — symmetrical small-joint arthritis (like rheumatoid arthritis), especially women.
  • Aplastic crisis — in people with haemolytic anaemias (sickle cell disease, hereditary spherocytosis) — sudden severe anaemia with very low reticulocytes.
  • Chronic anaemia in the immunosuppressed (pure red cell aplasia) — IVIG.
  • Pregnancy (especially before 20 weeks) — fetal anaemia and hydrops, miscarriage — test pregnant women exposed to a rash illness (IgG and IgM); fetal medicine referral if infected (intrauterine transfusion).
• HHV-6 (and HHV-7) — roseola infantum: high fever for 3–5 days in a young child, then a pink rash as the fever settles; febrile seizures.
• Enteroviruses (coxsackie viruses, echoviruses, enterovirus 71):
  • Hand, foot and mouth disease — mouth ulcers and vesicles on hands and feet (young children; nail shedding weeks later).
  • Herpangina — vesicles and ulcers on the soft palate.
  • Viral meningitis (commonest cause, summer and autumn), myocarditis and pericarditis (coxsackie B), Bornholm disease (painful chest wall muscle spasms), severe neonatal sepsis.
• HTLV-1 (blood-borne retrovirus — Caribbean, Japan, West Africa) — adult T-cell leukaemia/lymphoma, tropical spastic paraparesis, and severe Strongyloides infection.

Mpox

• Mpox (formerly monkeypox) — an orthopoxvirus; spread by close skin-to-skin contact (including sexual contact), contaminated bedding, and respiratory droplets with prolonged close contact.
• Clade II — caused the global outbreak from 2022, mainly among gay, bisexual and other men who have sex with men; clade I — more severe, with outbreaks in Central and East Africa; cases outside Africa need specialist management and public health input.
• Features — fever, headache, muscle aches and swollen lymph nodes, then skin lesions (papules → vesicles → pustules, often umbilicated, then crusts), often genital, perianal or oral; proctitis (severe rectal pain), sore throat; lesions may be few.
• Diagnosis — PCR swab of a lesion (tell the lab); test for HIV and other STIs.
• Management — isolation until lesions have healed; pain relief; most recover in 2–4 weeks; antivirals (tecovirimat) for severe disease or immunosuppression (specialist); notify.
• Prevention — MVA-BN vaccine for those at higher risk (offered through sexual health services) and as post-exposure vaccination for contacts.

Fever and Rash — Viral Causes at a Glance

• Don't miss non-viral causes — meningococcal septicaemia (non-blanching rash), toxic shock (erythroderma), scarlet fever (sandpaper rash, strawberry tongue), Kawasaki disease (children — fever 5 days or more), drug reactions (DRESS, SJS), secondary syphilis (palms and soles), rickettsial infection (eschar after travel).
• Rash illness in pregnancy — test for measles, rubella, parvovirus and VZV as appropriate (UKHSA rash guidance); keep away from antenatal clinics.

Candida Infections

• Superficial infections (common):
  • Oral thrush — white plaques that can be scraped off; dentures, inhaled steroids, antibiotics, diabetes, immunosuppression, frailty — nystatin or miconazole gel (raises INR with warfarin), or fluconazole.
  • Oesophageal candidiasis — painful swallowing; advanced HIV, steroids — fluconazole.
  • Vulvovaginal candidiasis — itch, thick white discharge — clotrimazole pessary or single-dose oral fluconazole 150 mg (not in pregnancy — topical only); recurrent — check for diabetes, longer suppressive courses.
  • Skin — intertrigo (skin folds), nappy rash, paronychia, angular cheilitis — topical azoles.
• Invasive candidiasis and candidaemia — ICU stay, central lines, parenteral nutrition, abdominal surgery, broad-spectrum antibiotics, neutropenia, dialysis:
  • Candida in blood is never a contaminant — echinocandin, remove lines, echo, eye examination (see page 1).
  • Deep sites — endocarditis, endophthalmitis, intra-abdominal abscesses, osteomyelitis.
• Candida auris — an emerging, multidrug-resistant yeast that spreads between patients and survives on surfaces; causes outbreaks in ICUs; screening of patients from affected hospitals, strict contact precautions, specialist treatment.
• Candida in sputum or a catheter urine is usually colonisation — don't treat routinely.

Aspergillus

Mucormycosis, Pneumocystis and Endemic Fungi

• Mucormycosis (Mucor, Rhizopus):
  • Risk factors — diabetic ketoacidosis and poorly controlled diabetes, neutropenia, high-dose steroids, iron overload (desferrioxamine), transplant.
  • Rhino-orbito-cerebral form — facial pain and swelling, nasal blockage, black necrotic eschar on the palate or nose, proptosis, eye movement problems, visual loss — spreads to the brain within days.
  • Also lung, skin and GI forms.
  • Emergency — urgent surgical debridement, high-dose liposomal amphotericin, correct DKA and reduce immunosuppression; isavuconazole or posaconazole to step down. Voriconazole doesn't work.
• Pneumocystis jirovecii pneumonia — not only in HIV: also in people on high-dose steroids plus other immunosuppression, after transplant, with haematological cancers and on some chemotherapy (e.g. fludarabine, rituximab regimens) — often more abrupt and severe than in HIV; co-trimoxazole prophylaxis for at-risk patients (see page 3).
• Cryptococcus — meningitis in advanced HIV, transplant recipients and steroid users (see page 3); pulmonary nodules in some healthy people.
• Endemic (geographically restricted) fungi — ask about travel and activities:
  • Histoplasmosis — bat and bird droppings, caves (Americas, Africa, Asia).
  • Coccidioidomycosis — desert dust (south-western USA, Mexico).
  • Blastomycosis, paracoccidioidomycosis, talaromycosis (South-East Asia — HIV).
  • Can cause flu-like illness, pneumonia, or disseminated disease in immunosuppression (sometimes years later); itraconazole or amphotericin.
• Dermatophytes (tinea) — see the Skin Infections and Infestations chapter.

Toxoplasmosis

• Toxoplasma gondii — a protozoan from cat faeces (soil, litter trays, unwashed vegetables) and undercooked meat; lifelong latent cysts.
• Healthy people — usually no symptoms, or a glandular fever-like illness with lymphadenopathy (often neck); self-limiting.
• Ocular toxoplasmosis — retinochoroiditis (floaters, blurred vision; "headlight in fog" appearance) — ophthalmology.
• Pregnancy — primary infection can pass to the fetus (risk higher later in pregnancy, but damage worse early):
  • Congenital toxoplasmosis — hydrocephalus, intracranial calcification, chorioretinitis, developmental delay.
  • Prevention advice — cook meat thoroughly, wash fruit and vegetables, wear gloves for gardening and changing cat litter (or avoid it).
  • Treatment of maternal infection — spiramycin (specialist), with fetal assessment.
• Immunosuppressed people — reactivation causes encephalitis (HIV with CD4 under 100 — multiple ring-enhancing lesions — see page 3), and infection after heart transplant from a positive donor; pyrimethamine plus sulfadiazine plus folinic acid; co-trimoxazole prophylaxis.

Giardia, Cryptosporidium and Amoebiasis

• Giardia — streams and contaminated water, travel, nurseries, men who have sex with men; prolonged watery diarrhoea, bloating, wind, sulphurous burps, weight loss and lactose intolerance; stool antigen or PCR; metronidazole or tinidazole; treat symptomatic household contacts.
• Cryptosporidium — swimming pools, farm animals (lambs, calves), contaminated water; watery diarrhoea for 1–2 weeks; severe and prolonged in immunosuppressed people (HIV, transplant); usually supportive; avoid swimming pools for 2 weeks after symptoms stop.
• Amoebiasis (Entamoeba histolytica — travel to the tropics):
  • Amoebic dysentery — gradual-onset bloody diarrhoea with abdominal pain (can mimic IBD — steroids for misdiagnosed "colitis" can cause perforation); stool PCR or microscopy.
  • Amoebic liver abscess — fever, right upper quadrant pain, tender liver, usually a single abscess in the right lobe; may occur months or years after travel, often without diarrhoea; ultrasound and amoebic serology.
  • Treatment — metronidazole (or tinidazole) followed by a luminal agent (paromomycin) to clear cysts from the gut; drainage rarely needed.
• Other gut protozoa — Cyclospora (travel, fresh produce — co-trimoxazole), Blastocystis and Dientamoeba (often harmless — treat only if symptomatic and no other cause).

Worms

• Eosinophilia after travel — think of worms (Strongyloides, schistosomiasis, hookworm, filariasis); send serology and stool microscopy; refer to a tropical or infection clinic.

Strongyloides

• Strongyloides stercoralis — larvae enter through the skin from soil in tropical and subtropical areas (including parts of southern Europe); it can complete its life cycle inside the body (autoinfection), so infection can persist for decades after leaving the area (e.g. migrants, former prisoners of war).
• Chronic infection — often no symptoms; intermittent diarrhoea, abdominal pain, cough, eosinophilia, larva currens (a fast-moving itchy linear rash on the buttocks or trunk).
• Hyperinfection syndrome — massive multiplication when immunity is suppressed, especially by corticosteroids (even short courses), HTLV-1 infection, transplant drugs and chemotherapy:
  • Severe GI and respiratory symptoms, and larvae carry gut bacteria → Gram-negative sepsis and meningitis; high mortality.
  • Eosinophilia may be absent (suppressed by steroids).
• Screen before immunosuppression — Strongyloides serology in people from (or who have lived in) endemic areas before high-dose steroids, transplant, biologics or chemotherapy; treat positives with ivermectin; in urgent situations (e.g. high-dose steroids for COVID-19 or severe asthma), empirical ivermectin may be given.

💡 Clinical Pearl: Gram-negative meningitis or sepsis in a patient on steroids who grew up in the tropics — think of Strongyloides hyperinfection, and screen before starting immunosuppression.

Prion Disease

• Prions — abnormally folded proteins that cause other proteins to misfold; not destroyed by normal sterilisation; always fatal.
• Sporadic CJD (most cases) — older adults; rapidly progressive dementia over weeks to months, myoclonus, ataxia, visual problems, akinetic mutism; death usually within a year.
• Variant CJD — linked to BSE ("mad cow disease") in beef; younger people; early psychiatric symptoms and painful sensory symptoms, then ataxia and dementia.
• Other forms — inherited (PRNP mutations), iatrogenic (contaminated surgical instruments, dura grafts, human-derived growth hormone).
• Investigations — MRI (DWI — cortical ribboning, basal ganglia or thalamic changes), CSF RT-QuIC test and 14-3-3 protein, EEG (periodic sharp-wave complexes in sporadic CJD); refer to the National CJD Research and Surveillance Unit.
• Infection control — special instrument handling for high-risk procedures (neurosurgery, eye surgery); notify; no treatment — supportive and palliative care.

Drug Reference Table