Infectious Diseases Revision Guide ✅

🦠 Infectious diseases is pattern recognition plus microbiology, host factors and source control. Start with: where is the infection?, how sick is the patient?, what organisms are likely?, what samples are needed?, and does this need antibiotics, antivirals, drainage, isolation or public health action? For exams and ward work, always connect the syndrome to the bug: pneumonia = respiratory pathogens, meningitis = time-critical CNS infection, cellulitis = skin flora, UTI = enteric Gram-negatives, and fever after travel = malaria until proven otherwise.

✅ 1. Core Infectious Disease Principles

🦠 1.1 Host, Bug and Site

• Host: age, pregnancy, immune status, diabetes, CKD, liver disease, HIV risk, splenectomy, neutropenia, devices and recent healthcare exposure.
• Bug: bacterial, viral, fungal, parasitic or mycobacterial.
• Site: respiratory, urinary, skin/soft tissue, CNS, GI, bone/joint, blood, heart valve, line/device.
• Severity: stable outpatient infection versus sepsis, shock, organ dysfunction or need for source control.
• Resistance risk: recent antibiotics, previous resistant organisms, hospitalisation, travel, care home, indwelling catheter/line.

🧪 1.2 Key Microbiology Samples

• Blood cultures: before antibiotics if this does not delay treatment; important in sepsis, endocarditis, line infection, osteomyelitis.
• Urine culture: pyelonephritis, complicated UTI, pregnancy, male UTI, recurrent UTI, catheter-associated UTI.
• Sputum culture: severe pneumonia, recurrent infection, bronchiectasis, immunosuppression.
• CSF: suspected meningitis/encephalitis if safe; do not delay antibiotics in unstable patients.
• Stool culture/PCR: bloody diarrhoea, severe/prolonged diarrhoea, travel, outbreaks, immunocompromise.
• Wound/abscess samples: ideally pus or deep tissue, not superficial swabs alone.
• Viral PCR: respiratory viruses, HSV/VZV, EBV/CMV, HIV depending on syndrome.

🧠 Exam pearl: The best culture is from the infected site. A superficial swab from a chronic ulcer often grows colonisers, while pus, tissue, blood or sterile-site samples are much more clinically useful.

🛡️ 1.3 Infection Prevention Basics

• Hand hygiene is the highest-yield infection control intervention.
• Use standard precautions for all patients: hand hygiene, PPE by exposure risk, safe sharps and cleaning.
• Use transmission-based precautions when needed: contact, droplet or airborne isolation.
• Screening and decolonisation may be used for MRSA in selected surgical/high-risk pathways.
• Good catheter, cannula and line care prevents healthcare-associated infection.

🚨 2. Sepsis

Sepsis is life-threatening organ dysfunction caused by a dysregulated host response to infection. NICE NG253 now covers suspected sepsis in people aged 16 or over who are not and have not recently been pregnant, replacing older NG51 content for this group.

⚡ 2.1 Recognising Sepsis

• Possible sources: pneumonia, UTI/pyelonephritis, abdominal sepsis, skin/soft tissue infection, meningitis, line infection, endocarditis.
• Red flags: new confusion, mottled/ashen skin, hypotension, hypoxia, tachypnoea, reduced urine output, high lactate, non-blanching rash.
• Older adults may present with falls, delirium, reduced mobility or reduced oral intake rather than fever.
• Immunosuppressed patients may have minimal fever or inflammatory signs.
• Pregnancy/recent pregnancy and under-16s have separate NICE sepsis guidance pathways.

🧯 2.2 Initial Management Principles

• ABCDE assessment and early senior help.
• Oxygen if hypoxic and ventilatory support if needed.
• Blood cultures and relevant site cultures if this does not delay urgent treatment.
• Prompt antibiotics for high-risk/severely unwell patients according to local policy.
• IV fluids if hypoperfusion/shock, with careful reassessment to avoid overload.
• Check lactate, FBC, U&E, LFT, clotting, glucose, VBG/ABG and urine output.
• Find and control the source: drain pus, remove infected line, decompress obstruction, operate if needed.

🚨 Safety pearl: Antibiotics treat organisms, but source control treats the reason the patient remains septic. An abscess, obstructed infected kidney or perforated bowel needs mechanical control.

💊 3. Antimicrobial Stewardship

NICE NG15 covers antimicrobial stewardship systems and processes for effective antimicrobial use. The aim is to treat infection effectively while slowing antimicrobial resistance and reducing avoidable harm.

🎯 3.1 Start Smart, Then Focus

• Start antibiotics promptly when infection is likely and risk is high.
• Use local guidelines based on likely source, severity and resistance patterns.
• Take cultures before antibiotics where safe and practical.
• Document indication, dose, route, duration/review date and allergy status.
• Review at 48–72 hours: stop, narrow, switch IV-to-oral, continue or escalate.
• Use the narrowest effective agent once organism and sensitivities are known.
• Remove unnecessary devices such as catheters and cannulas.

⚠️ 3.2 Antibiotic Safety

🧠 Exam pearl: “Broad-spectrum” is not automatically better. The best antibiotic is the one that covers likely pathogens, penetrates the site, fits patient factors and can be narrowed safely.

🫁 4. Respiratory Infections

NICE NG250, published in 2025, covers diagnosis and management of community-acquired and hospital-acquired pneumonia across children and adults, replacing older NICE pneumonia antimicrobial guidance.

🦠 4.1 Community-Acquired Pneumonia

• Features: fever, cough, sputum, pleuritic chest pain, dyspnoea, tachypnoea, hypoxia, crackles/bronchial breathing.
• Older adults may present with confusion, falls, functional decline or reduced intake.
• Common organisms: Streptococcus pneumoniae, Haemophilus influenzae, atypicals and respiratory viruses.
• Assess severity using clinical judgement and scores such as CRB-65/CURB-65 where appropriate.
• Management: oxygen if hypoxic, antibiotics guided by severity/local policy, fluids, analgesia and reassessment.

🏥 4.2 Hospital-Acquired Pneumonia

• Occurs 48 hours or more after hospital admission.
• Higher risk of Gram-negative organisms, Staphylococcus aureus and resistant pathogens.
• Risk factors: immobility, aspiration, poor oral care, dysphagia, recent antibiotics, ventilation.
• Management depends on severity, local microbiology and resistance risk.

🫧 4.3 Viral Respiratory Infection

• Influenza: fever, myalgia, headache, cough; severe in older adults, pregnancy, chronic disease and immunosuppression.
• COVID-19: variable respiratory/systemic illness; consider risk stratification and local antiviral pathways.
• RSV: important in infants, older adults and chronic lung/heart disease.
• Antibiotics do not treat viral infection, but bacterial coinfection may occur.
• Vaccination reduces severe disease and transmission risk in eligible groups.

🧱 4.4 Lung Abscess and Empyema

• Lung abscess: necrotising infection, often aspiration-related; foul sputum, fever, weight loss, cavitating lesion.
• Empyema: infected pleural fluid; persistent fever despite pneumonia treatment, pleuritic pain, raised inflammatory markers.
• Pleural fluid pH <7.2, low glucose or frank pus suggests need for drainage.
• Source control with chest drain/surgery may be required.

🫘 5. Urinary Tract Infection

🚽 5.1 Lower UTI and Pyelonephritis

• Lower UTI: dysuria, frequency, urgency, suprapubic pain, haematuria.
• Pyelonephritis: fever, rigors, loin pain, nausea/vomiting, systemic illness.
• Complicated UTI: male sex, pregnancy, catheter, obstruction, renal tract abnormality, immunosuppression, sepsis.
• Urine culture is important in pyelonephritis, recurrent infection, pregnancy, male UTI and treatment failure.
• Antibiotic choice depends on site, severity, pregnancy, renal function and local resistance.

🧫 5.2 Catheter-Associated UTI

• Catheters frequently cause bacteriuria; do not treat asymptomatic bacteriuria unless specific indications apply.
• Features suggesting infection: fever, suprapubic/loin pain, rigors, delirium with no other cause, sepsis.
• Change or remove catheter where appropriate before culture/treatment.
• Prevention: avoid unnecessary catheters, aseptic insertion, closed drainage and early removal.

🚨 5.3 Infected Obstructed Kidney

• Sepsis plus urinary obstruction/hydronephrosis or stone is a urological emergency.
• Features: fever, rigors, loin pain, AKI, hypotension, pyuria/positive culture.
• Management: IV antibiotics, fluids and urgent decompression by ureteric stent or nephrostomy.

🧠 6. CNS Infection

🚨 6.1 Meningitis

• Features: fever, headache, neck stiffness, photophobia, vomiting, confusion, seizures, non-blanching rash.
• Infants and older adults may present atypically.
• Important organisms: Neisseria meningitidis, Streptococcus pneumoniae, Listeria in older/immunosuppressed/pregnancy risk groups, viruses.
• Do not delay antibiotics if bacterial meningitis or meningococcal sepsis is suspected.
• CT before lumbar puncture if reduced consciousness, focal neurology, papilloedema, seizures, immunosuppression or raised ICP concern.
• Give adjunctive dexamethasone according to local protocol when bacterial meningitis suspected.

🧠 6.2 Encephalitis

• Brain inflammation causing altered mental state, seizures, focal neurology or behavioural change.
• HSV encephalitis is time-critical and often affects temporal lobes.
• Start IV aciclovir promptly if suspected; do not wait for CSF PCR if clinical concern is high.
• Autoimmune encephalitis can mimic infection or psychiatric illness.

🧠 6.3 Brain Abscess

• Sources: sinusitis, otitis media, dental infection, endocarditis, neurosurgery, immunosuppression.
• Features: headache, fever, seizure, focal neurology, raised ICP; full triad may be absent.
• Management: urgent imaging, blood cultures, IV antibiotics and neurosurgical drainage depending on size/site.

🚨 Safety pearl: In suspected meningitis/sepsis, lumbar puncture is never allowed to delay life-saving antibiotics.

🩹 7. Skin, Soft Tissue, Bone and Joint Infection

🔥 7.1 Cellulitis and Erysipelas

• Cellulitis: deeper dermis/subcutaneous infection; red, hot, swollen, painful, usually unilateral.
• Erysipelas: more superficial with sharply demarcated raised edge.
• Common organisms: Streptococcus pyogenes and Staphylococcus aureus.
• Risk factors: skin breaks, tinea pedis, ulcers, oedema, lymphoedema, obesity, diabetes.
• Bilateral red legs are more often venous eczema/oedema than bilateral cellulitis.

🚨 7.2 Necrotising Fasciitis

• Life-threatening deep soft tissue infection requiring urgent surgery.
• Red flags: pain out of proportion, rapid progression, systemic toxicity, bullae, skin necrosis, crepitus, anaesthesia over skin.
• Management: immediate surgical review, broad-spectrum IV antibiotics, resuscitation and debridement.
• Do not delay surgery for imaging if clinical suspicion is high.

🦴 7.3 Osteomyelitis

• Bone infection from haematogenous spread, contiguous infection or direct inoculation.
• Risk factors: diabetes, foot ulcer, vascular disease, trauma, prosthetic material, IV drug use.
• Features: local bone pain, fever, swelling, raised inflammatory markers; chronic disease may be subtle.
• MRI is sensitive for diagnosis; bone biopsy/culture helps target therapy.
• Management often needs prolonged antibiotics and sometimes surgical debridement.

🦵 7.4 Septic Arthritis

• Hot swollen painful joint with restricted movement is septic arthritis until proven otherwise.
• Risk factors: prosthetic joint, RA, diabetes, immunosuppression, older age, IV drug use.
• Diagnosis: urgent joint aspiration for Gram stain, culture, cell count and crystals.
• Crystals do not exclude infection.
• Management: IV antibiotics and joint washout/drainage with orthopaedic/rheumatology input.

🚨 Exam pearl: Never inject steroids into a hot swollen joint until septic arthritis has been excluded.

🍽️ 8. Gastrointestinal and Hepatobiliary Infection

🚽 8.1 Infectious Diarrhoea

• Common pathogens: norovirus, Campylobacter, Salmonella, Shigella, E. coli, Giardia and Cryptosporidium.
• Bloody diarrhoea suggests invasive bacterial infection, IBD or ischaemic colitis.
• Send stool culture/PCR if severe, bloody, prolonged, travel-related, outbreak-related, immunocompromised or hospital-acquired.
• Most cases need fluids and safety-netting rather than antibiotics.
• Avoid antimotility drugs in suspected dysentery or severe colitis.

🦠 8.2 C. difficile Infection

• Risk factors: antibiotics, older age, hospitalisation, care home, immunosuppression and PPI use.
• Features: watery diarrhoea, abdominal pain, fever, leukocytosis; severe disease can cause toxic megacolon.
• Management: isolate, stop unnecessary antibiotics/PPIs, treat with appropriate anti-C. difficile therapy, monitor severity.
• Recurrent infection may need specialist strategies such as fidaxomicin or faecal microbiota transplant pathways.

🟡 8.3 Cholangitis

• Infection of obstructed biliary tree.
• Charcot triad: fever, jaundice, right upper quadrant pain.
• Reynolds pentad adds hypotension and confusion.
• Management: ABCDE, blood cultures, IV antibiotics, fluids and urgent biliary drainage, usually ERCP.

🧫 8.4 Liver Abscess

• Pyogenic liver abscess: fever, RUQ pain, sepsis, raised ALP/CRP; often biliary or portal source.
• Amoebic liver abscess: travel/endemic exposure, fever, RUQ pain; Entamoeba histolytica.
• Management: imaging, cultures/serology, antibiotics/antiparasitic therapy and drainage depending on size/response.

❤️ 9. Bloodstream, Line and Endocarditis Infection

🩸 9.1 Bloodstream Infection

• Bacteraemia may arise from urinary, respiratory, abdominal, skin/soft tissue, line or endocarditis sources.
• Staphylococcus aureus bacteraemia is high-risk and needs source search, repeat cultures and specialist input.
• Persistent bacteraemia suggests uncontrolled source, endocarditis, infected thrombus, abscess or infected device.
• Remove infected lines/devices where appropriate and safe.

❤️ 9.2 Infective Endocarditis

• Infection of endocardial surface, usually heart valves, forming vegetations.
• Risk factors: valve disease, prosthetic valve, previous IE, congenital heart disease, IV drug use, cardiac devices, haemodialysis.
• Features: fever, murmur, emboli, stroke, back pain, splenomegaly, haematuria, Osler nodes, Janeway lesions, splinter haemorrhages.
• Take 3 sets of blood cultures from separate sites before antibiotics if stable.
• Echo: TTE first in many cases; TOE is more sensitive, especially prosthetic valves or complications.
• Management requires prolonged IV antibiotics and sometimes surgery for heart failure, abscess, persistent infection or embolic risk.

🧵 9.3 Line Infection

• Features: fever/rigors during line use, exit-site infection, positive blood cultures, no other source.
• Central lines, dialysis lines and long-term IV access devices are high-risk.
• Management may require paired cultures, line removal, antibiotics and specialist advice.

🧬 10. HIV and Opportunistic Infection

🧪 10.1 HIV Basics

• HIV infects CD4 T cells, causing progressive immune dysfunction without treatment.
• Transmission: sexual, blood exposure, vertical transmission, needle sharing.
• Acute seroconversion: fever, rash, sore throat, lymphadenopathy, myalgia, aseptic meningitis-like illness.
• Offer HIV testing in indicator conditions, high-prevalence settings, STI diagnosis, TB, hepatitis B/C and unexplained immunosuppression.
• Modern antiretroviral therapy can suppress viral load and preserve immune function.
• Undetectable viral load means effectively no sexual transmission risk: U=U.

🦠 10.2 Opportunistic Infections

🫁 10.3 Pneumocystis Pneumonia

• Features: subacute dry cough, dyspnoea, fever, hypoxia, desaturation on exertion.
• CXR may show bilateral interstitial infiltrates but can be normal early.
• High LDH may support but is not diagnostic.
• Treatment: high-dose co-trimoxazole; steroids if significant hypoxia, according to specialist guidance.

⚠️ Clinical pearl: HIV testing is part of good medical care, not a moral judgement. Test when clinically indicated and explain it routinely.

🫁 11. Tuberculosis

NICE NG33 covers prevention, identification and management of latent and active TB in children, young people and adults. It was last reviewed by NICE in February 2024.

🧫 11.1 Pulmonary TB

• Features: cough lasting weeks, haemoptysis, fever, night sweats, weight loss, fatigue.
• Risk factors: close contact, birth/residence/travel in high-incidence area, homelessness, prison, immunosuppression, HIV, diabetes.
• CXR: upper zone infiltrates/cavitation, miliary pattern or lymphadenopathy.
• Diagnosis: sputum AFB smear/culture, NAAT/PCR, imaging and public health assessment.
• Treatment uses combination therapy for months; adherence and drug toxicity monitoring are essential.
• TB is notifiable and requires contact tracing.

🧠 11.2 Extrapulmonary TB

• Can affect lymph nodes, bone/spine, CNS, pericardium, abdomen, genitourinary tract and disseminated systems.
• Spinal TB/Pott disease can cause back pain, deformity and cord compression.
• TB meningitis causes subacute headache, fever, cranial nerve palsies and confusion.
• Diagnosis often needs biopsy/fluid sampling for AFB culture/PCR and histology.

🌙 11.3 Latent TB

• Latent TB infection means immune containment without active disease symptoms.
• Screening uses IGRA and/or tuberculin skin testing depending on pathway.
• Treating latent TB reduces future active TB risk in selected groups.
• Exclude active TB before starting latent TB treatment.

✈️ 12. Travel and Tropical Infection

🦟 12.1 Fever in the Returning Traveller

• Malaria must be considered in any febrile traveller from an endemic region, even with prophylaxis.
• Ask: destination, dates, rural/urban, mosquito exposure, freshwater exposure, animal bites, food/water, sexual exposure, healthcare exposure, vaccinations and prophylaxis.
• Urgent tests: malaria thick/thin films or rapid test, FBC, U&E, LFT, CRP, blood cultures and other tests by geography/syndrome.
• Red flags: bleeding, shock, jaundice, confusion, respiratory distress, renal failure or severe thrombocytopenia.

🦟 12.2 Malaria

• Features: fever, rigors, sweats, headache, myalgia, GI symptoms; may be periodic but often irregular early.
• Falciparum malaria can progress rapidly and is life-threatening.
• Bloods: thrombocytopenia, anaemia, raised bilirubin/LFTs, AKI, hypoglycaemia, acidosis in severe disease.
• Diagnosis requires urgent malaria testing; repeat testing if initial negative and suspicion remains.
• Severe malaria needs urgent specialist/infectious diseases input and IV antimalarial therapy.

🦠 12.3 Dengue, Enteric Fever and Rickettsial Disease

• Dengue: fever, severe myalgia, retro-orbital pain, rash, thrombocytopenia; avoid NSAIDs due to bleeding risk.
• Enteric fever: prolonged fever, abdominal symptoms, relative bradycardia, exposure to contaminated food/water.
• Rickettsial disease: fever, headache, rash/eschar after tick/mite exposure; doxycycline often used where appropriate.

🚨 Exam pearl: Fever after travel to a malaria region is malaria until proven otherwise. A single negative test may not be enough if suspicion remains.

💉 13. Viral Hepatitis and Blood-Borne Viruses

🟡 13.1 Hepatitis A and E

• Usually faeco-oral transmission.
• Features: fever, malaise, nausea, RUQ pain, jaundice, dark urine, pale stools.
• Hepatitis E can be severe in pregnancy and immunosuppression.
• Management is usually supportive, with public health advice and monitoring.

🩸 13.2 Hepatitis B

• Transmission: blood, sexual, vertical, needle exposure.
• Can cause acute hepatitis or chronic infection with cirrhosis/HCC risk.
• Serology interpretation is high yield: HBsAg indicates current infection; anti-HBs indicates immunity; anti-HBc indicates previous exposure/current infection depending on IgM/IgG.
• Vaccination prevents infection; post-exposure prophylaxis depends on exposure and immune status.

🩸 13.3 Hepatitis C

• Commonly transmitted by blood exposure, especially injecting drug use.
• Often asymptomatic for years, causing chronic hepatitis, cirrhosis and HCC risk.
• Test with antibody and confirm active infection with HCV RNA.
• Direct-acting antivirals cure most infections and are specialist-led.

🧪 13.4 Needlestick Injury

• Immediate first aid: encourage bleeding, wash with soap/water, cover wound; do not scrub aggressively.
• Assess source risk for HIV, HBV, HCV and exposure type.
• Urgent occupational health/ED pathway for HIV PEP consideration, HBV vaccination/immunoglobulin and baseline/follow-up tests.
• HIV PEP is time-critical and should be started as soon as possible when indicated.

🧫 14. Sexually Transmitted Infections

• Take a non-judgemental sexual history: partners, practices, protection, past STIs, pregnancy risk and safeguarding.
• Offer appropriate NAAT testing, HIV/syphilis/hepatitis testing and partner notification.
• Consider PID in pelvic pain with cervical excitation/adnexal tenderness.
• Assess for sexual assault, coercion, exploitation and domestic abuse where relevant.

🧯 15. Infections in Special Hosts

🧬 15.1 Neutropenic Sepsis

• Fever or sepsis in neutropenia is a medical emergency.
• Common after chemotherapy or marrow failure.
• Patients may lack localising signs because neutrophils drive pus/inflammation.
• Give empirical IV antibiotics urgently according to local neutropenic sepsis protocol.
• Look for line infection, mucositis, pneumonia, abdominal sepsis, skin and perianal sources.

🛡️ 15.2 Asplenia and Hyposplenism

• Higher risk of overwhelming infection with encapsulated organisms: pneumococcus, meningococcus, Hib.
• Causes: splenectomy, sickle cell disease, coeliac disease, splenic infarction.
• Prevention: vaccination, antibiotic prophylaxis where indicated, emergency antibiotics advice and medical alert information.
• Fever in asplenia is high risk and needs urgent assessment.

🤰 15.3 Infection in Pregnancy

• Some infections have fetal risk: rubella, varicella, parvovirus B19, CMV, toxoplasmosis, syphilis, HIV, hepatitis B, listeria.
• UTI/pyelonephritis risk is higher and treatment choices differ.
• Sepsis can deteriorate rapidly; pregnancy/recent pregnancy has separate NICE sepsis guidance.
• Always check pregnancy status when prescribing antimicrobials in reproductive-age patients.

🧓 15.4 Infection in Older Adults

• Fever may be absent.
• Presentations include delirium, falls, functional decline, anorexia or reduced mobility.
• Avoid diagnosing UTI from positive urine dip alone in the absence of urinary symptoms or systemic infection evidence.
• Consider aspiration pneumonia, skin/soft tissue infection, biliary sepsis, catheter infection and C. difficile.

🚨 16. Infectious Disease Emergencies

📚 17. OSCE / Exam Pearls

• Always identify the infection source: chest, urine, abdomen, skin, CNS, line, bone/joint or heart valve.
• Take cultures before antibiotics if safe, but do not delay antibiotics in severe sepsis or meningitis.
• Fever after travel to a malaria area is malaria until proven otherwise.
• Neutropenic sepsis can look deceptively mild.
• Non-blanching rash plus fever is meningococcaemia until proven otherwise.
• Hot swollen joint is septic arthritis until proven otherwise.
• Pain out of proportion suggests necrotising fasciitis or ischaemia.
• Persistent Staphylococcus aureus bacteraemia should prompt source/endocarditis search.
• Antibiotic review at 48–72 hours is core antimicrobial stewardship.
• Source control often matters more than changing antibiotics repeatedly.

📌 18. Quick Differentials Table

📚 References

• NICE. Suspected sepsis in people aged 16 or over: recognition, diagnosis and early management. NG253.
• NICE. Suspected sepsis in under 16s: recognition, diagnosis and early management. NG254.
• NICE. Suspected sepsis in people who are pregnant or have recently been pregnant. NG255.
• NICE. Antimicrobial stewardship: systems and processes for effective antimicrobial medicine use. NG15.
• NICE. Pneumonia: diagnosis and management. NG250.
• NICE. Tuberculosis. NG33.
• UKHSA, BASHH, BHIVA, British Infection Association and local antimicrobial guidelines should be checked for specialist infection, STI, HIV, travel, outbreak and resistance pathways.

⚠️ Disclaimer

This article is for medical education and exam revision. Clinical decisions should follow current local antimicrobial guidelines, sepsis pathways, infection prevention policies, public health requirements, microbiology advice, formularies, senior advice and national guidance. Infectious disease emergencies such as sepsis, meningitis, encephalitis, necrotising fasciitis, septic arthritis, neutropenic sepsis, severe malaria, cholangitis and infected obstruction require urgent senior input.