Infections can be classified based on different criteria: causative agent, clinical presentation, anatomical location, mode of transmission, and duration.

• Bacterial – *Streptococcus pneumoniae*, *Escherichia coli*, *Mycobacterium tuberculosis*
• Viral – *Influenza virus*, *Herpes simplex virus*, *HIV*
• Fungal – *Candida albicans*, *Cryptococcus neoformans*
• Parasitic – *Plasmodium falciparum*, *Toxoplasma gondii*
• Prion-related – *Creutzfeldt-Jakob disease*

• Respiratory infections – Pneumonia, bronchitis, sinusitis
• Central nervous system infection(CNS) – Meningitis, encephalitis, brain abscess
• Gastrointestinal – Gastroenteritis, hepatitis
• Genitourinary – Cystitis, pyelonephritis, STIs
• Skin and soft tissue – Cellulitis, abscesses
• Bone and joint – Osteomyelitis, septic arthritis

• Direct contact – Herpes, fungal skin infections
• Airborne / Droplet – Influenza, COVID-19, TB
• Fecal-oral – Hepatitis A, cholera
• Vector-borne – Malaria, dengue
• Blood-borne / Sexual – HIV, hepatitis B & C

• Acute – Rapid onset and short duration (e.g., influenza)
• Subacute – Intermediate duration (e.g., subacute bacterial endocarditis)
• Chronic – Long-lasting (e.g., tuberculosis, hepatitis C)
• Latent – Present but asymptomatic, with potential reactivation (e.g., herpes viruses)

• Community-acquired – Contracted outside healthcare settings
• Hospital-acquired (nosocomial) – Acquired during hospitalization
• Opportunistic – In immunocompromised hosts (e.g., HIV-related infections)