Viruses are obligate intracellular parasites consisting of nucleic acid enclosed in a protein capsid; a complete infectious particle outside a host cell is called a virion. Enveloped viruses possess a host-derived lipid bilayer surrounding the capsid, while non-enveloped (naked) viruses consist only of a protein capsid and tend to be more resistant to desiccation and disinfectants. Capsid symmetry is either icosahedral or helical, with poxviruses exhibiting complex symmetry. DNA viruses typically replicate in the nucleus (poxviruses being the exception), whereas RNA viruses usually replicate in the cytoplasm, though influenza and retroviruses require nuclear steps. Bacteriophages can replicate through a lytic cycle, immediately producing progeny and lysing the host, or a lysogenic cycle, integrating their DNA into the host genome as a prophage that replicates passively until induced.
Retroviruses such as HIV use reverse transcriptase to convert their RNA genome into DNA. HIV targets CD4+ T helper lymphocytes, as well as macrophages and dendritic cells, by binding CD4 plus co-receptors CCR5 or CXCR4. Disease progression is monitored by CD4+ T cell count, with AIDS diagnosed when counts fall below 200 cells/microliter or when an AIDS-defining illness develops. The respiratory viruses include rhinoviruses (Picornaviridae), the most frequent cause of the common cold, and influenza virus, an orthomyxovirus bearing two major surface antigens, hemagglutinin (HA) and neuraminidase (NA). Antigenic drift involves gradual point mutations in HA and NA, producing seasonal epidemics, while antigenic shift is abrupt reassortment of gene segments between strains and can cause pandemics. Measles virus (Paramyxoviridae) produces characteristic Koplik spots on the buccal mucosa, and mumps virus causes parotitis with swollen salivary glands.
Herpesviruses establish lifelong latency in host cells with potential for reactivation. Varicella-zoster virus (VZV) causes chickenpox on primary infection and shingles upon reactivation. Epstein-Barr virus (EBV) causes infectious mononucleosis and is linked to Burkitt lymphoma and nasopharyngeal carcinoma. Cytomegalovirus (CMV) is associated with congenital hearing loss and severe disease in immunocompromised transplant patients. HSV-1 typically causes oral herpes, while HSV-2 typically causes genital herpes, though both can cause either. Hepatitis viruses differ in transmission and outcome: hepatitis A and E are fecal-oral and generally self-limited, with hepatitis E being particularly dangerous in pregnant women; hepatitis B, a hepadnavirus transmitted by blood and body fluids, can become chronic and increase liver cancer risk; hepatitis C (Flaviviridae) is the most common cause of chronic viral hepatitis in many countries and is transmitted mainly via blood; and hepatitis D is a defective virus requiring co-infection with hepatitis B.
Other medically important viruses include rabies virus (Rhabdaviridae), which produces characteristic Negri body cytoplasmic inclusions; rotavirus, the leading cause of viral gastroenteritis in children; and norovirus, the most common cause of outbreak-associated viral gastroenteritis on cruise ships and similar settings. Poliovirus (Picornaviridae) targets motor neurons in the anterior horn of the spinal cord, causing flaccid paralysis. Arboviruses transmitted by mosquitoes include yellow fever virus (Flaviviridae, vector Aedes mosquitoes), dengue virus, which causes fever, rash, and severe joint pain, and Zika virus, which is associated with congenital microcephaly when contracted during pregnancy. SARS-CoV-2 binds the ACE2 receptor via its spike protein to enter human cells. Vaccination strategies rely on either live-attenuated vaccines, using weakened replicating pathogens that elicit strong and durable immunity but carry small risk in immunocompromised hosts, or inactivated (killed) vaccines, which are safer but often require boosters and adjuvants.