Virology

Viruses are classified by their type of nucleic acid (DNA or RNA), capsid symmetry (icosahedral, helical, or complex), presence or absence of an envelope, and replication strategy. The Baltimore classification system groups viruses into seven classes based on how they produce messenger RNA from their genomes.

The series of steps by which a virus reproduces inside a host cell: attachment to specific receptors, entry into the cell, uncoating of the nucleic acid, replication of the genome, expression of viral proteins, assembly of new virions, and release by lysis or budding.

The innate and adaptive immune mechanisms that detect and eliminate viral infections. Innate responses include interferon production, natural killer cell activity, and inflammatory signaling. Adaptive responses involve virus-specific antibodies produced by B cells and cytotoxic T lymphocytes that kill infected cells.

Antigenic drift refers to gradual accumulation of mutations in viral surface proteins through replication errors, while antigenic shift involves the sudden reassortment of genome segments between different viral strains, producing a dramatically different surface antigen profile.

Biological preparations that stimulate the immune system to recognize and fight specific viruses. Types include live-attenuated vaccines, inactivated vaccines, subunit vaccines, viral vector vaccines, and nucleic acid vaccines (mRNA and DNA). Vaccines have eliminated or controlled many viral diseases.

The transmission of a virus from an animal reservoir to humans, often facilitated by ecological disruption, close human-animal contact, or intermediate hosts. Most emerging infectious diseases are zoonotic in origin, making wildlife surveillance critical for pandemic preparedness.

Pharmacological agents that target specific stages of the viral replication cycle. Classes include entry inhibitors, polymerase inhibitors, protease inhibitors, integrase inhibitors, and neuraminidase inhibitors. Unlike antibiotics, antivirals are typically virus-specific.

RNA viruses exist as diverse populations of closely related genetic variants called quasispecies rather than single defined sequences. High mutation rates, short generation times, and large population sizes drive rapid evolution, enabling adaptation to new hosts, immune evasion, and drug resistance.

Viruses that specifically infect bacteria. Phages are the most abundant biological entities on Earth and play critical roles in microbial ecology, horizontal gene transfer, and the regulation of bacterial populations. They are also being explored as alternatives to antibiotics in phage therapy.

The process by which certain viruses contribute to the development of cancer by inserting oncogenes into the host genome, inactivating tumor suppressor genes, or causing chronic inflammation. Approximately 15-20% of human cancers are associated with viral infections.