Virology

In 2020, a novel coronavirus spread across the world, and within weeks, clinical microbiologists had characterized its genome, developed PCR-based diagnostic tests, and begun evaluating serological assays for population-level surveillance. That speed was possible because the foundational principles of virology (viral structure, replication, tropism, and immune evasion) were already understood.

Virology is the study of viruses: obligate intracellular parasites that require a host cell to replicate, cause disease through mechanisms distinct from bacteria or fungi, and pose unique diagnostic challenges because they cannot be grown on standard bacteriological media.

This section covers:

• Viral structure and classification: capsid morphology, envelope composition, genome type (DNA vs. RNA, single- vs. double-stranded, segmented vs. non-segmented), and the Baltimore classification system
• Viral replication: attachment, entry, genome replication, assembly, and release; how antiviral drugs target specific steps in this cycle
• Organism profiles: all major clinically important virus families, including Herpesviridae, Hepatitis viruses, HIV, Influenza, Dengue, Measles, Rabies, HPV, Rotavirus, and others
• Pathogenesis and immune evasion: how viruses cause cell damage, establish latency, and evade host immune responses
• Laboratory diagnosis: cell culture, PCR-based detection, antigen testing, and serology; how to interpret IgM versus IgG results; the role of viral load testing in monitoring

A recurring theme in clinical virology is the interpretation of serological results, understanding that IgM indicates recent infection and IgG indicates past exposure or vaccination, and knowing when those rules have exceptions, is as important as memorizing which virus causes which disease.