Overview
It’s important to know how viruses are classified and the key diseases they cause. This is true for most viruses. However, for hepatitis viruses, influenza virus, and HIV, you also need to learn more detailed information about pathogenicity, transmission, and a few other features.
General virology: Viruses are the smallest obligate intracellular pathogens that infect humans. A virion is the complete infectious particle. It contains:
- A viral genome (either DNA or RNA)
- A protein coat called a capsid
- Sometimes a lipid envelope surrounding the capsid
The genome plus the capsid is called the nucleocapsid.
If a virus has an envelope, it contains envelope glycoproteins, which are important for attachment to the host cell surface. Because the envelope is lipid, it can be disrupted by ether, alcohols, phenols, and certain disinfectants. When the envelope is dissolved, enveloped viruses lose infectivity.
The capsid is made of repeating subunits called capsomeres. Capsomeres can be arranged with different types of symmetry:
- Icosahedral (a complex polygon)
- Helical (spiral-shaped)
- Complex (neither icosahedral nor helical)
Viruses are classified based on their genome and structure, including whether the genome is:
- Single-stranded or double-stranded
- DNA or RNA
- Segmented or unsegmented
- Circular or linear
Capsid morphology is also part of classification. This matters clinically because USMLE-style vignettes often describe viral morphology, and you’ll use those clues to identify the likely virus.
Viruses can be classified as below: