Viral Diseases

Earliest evidence of viral disease

Ancient Egyptian hieroglyph (ca. 3700 BC) showing poliomyelitis; smallpox in China (1000 BC); rabies painting (1224 CE).

Who pioneered vaccination and when?

Edward Jenner, 1796, used cowpox to prevent smallpox—before viruses were known.

What was Ivanovsky’s 1892 discovery?

Filtered sap from infected tobacco plants remained infectious—showed presence of an “invisible agent.”

Who coined the term contagium vivum fluidum?

Martinus Beijerinck, 1898—“contagious living fluid,” defining the infectious agent smaller than bacteria.

Who discovered the first animal virus?

Loeffler & Frosch, 1898—Foot-and-mouth disease virus in cattle.

Why were filters important in virology?

Chamberland porcelain filters removed bacteria; infectious filtrate proved the existence of smaller agents—birth of virology.

Rous’s discovery (1911)

Peyton Rous showed a filterable agent (Rous sarcoma virus) caused tumors in chickens.

Stanley’s contribution (1935)

Crystallized Tobacco Mosaic Virus (TMV) and showed it remained infectious—proved viruses have physical structure.

Who grew poliovirus in vitro and when?

Enders, Weller & Robbins, 1949—used human tissue culture; Nobel Prize 1954.

Discovery of reverse transcriptase

Temin & Baltimore, 1970 — in retroviruses; Nobel Prize 1975.

Discovery of proto-oncogenes

Bishop & Varmus, 1976 — showed RSV oncogene also in normal cells; Nobel Prize 1989.

When was smallpox eradicated?

Last case 1979; declared eradicated by WHO in 1980.

Recent viral discoveries

HTLV (1980), HIV (1981), HCV (1989), HHV-8 (1994), MERS-CoV (2012), SARS-CoV-2 (2020).

Proportion of diseases caused by viruses

~60 % of all infectious diseases.

Examples of common viral diseases

Common cold, influenza, MMR, chickenpox, AIDS, hepatitis, viral diarrhea, polio, COVID-19.

Global impact of HIV/AIDS (~2024)

~630,000 deaths; ~39 million people living with HIV globally.

Global impact of Hepatitis B & C (~2022)

~1.3 million deaths; >300 million affected worldwide.

Definition of a virus

“A piece of bad news wrapped up in a protein shell” – Peter Medawar (1960). Acellular, genome = DNA or RNA (never both), obligate intracellular parasite.

What is a virion?

Inert extracellular particle protecting viral genome; transports it between cells like a “spacecraft.”

How do viruses differ from cellular organisms?

Not made of cells; metabolically inert; replicate only inside host; no energy metabolism; no protein synthesis machinery.

Can viruses grow on sterile media?

No—require living host cells for replication.

Typical virus size range

20–300 nm (some up to 1000 nm); smaller than most bacteria but larger than prions.

Example of a giant virus

Mimiviridae (family of giant viruses, 700–1000 nm long).

Example of revived ancient virus

Pithovirus sibericum (30,000 years old, infects amoebas); shows risk from melting permafrost.

Tobacco Mosaic Virus (TMV) structure

Single RNA molecule in cylindrical protein coat; 300 nm long × 15 nm wide.

T4 bacteriophage structure

DNA in protein head with tail for injecting DNA into bacteria.

HIV structure

Retrovirus with RNA genome and host-derived lipid envelope; replicates via DNA intermediate.

Types of viral proteins

Non-structural (enzymes for replication); Structural (form capsid).

What is a nucleocapsid?

Viral genome + capsid (protein shell); core unit of the virus.

Capsid symmetry types

Icosahedral (spherical), Helical (tube-like), Complex (bacteriophages).

Difference between enveloped and non-enveloped viruses

Enveloped: lipid bilayer with glycoproteins from host. Non-enveloped: nucleocapsid only.

Environmental stability of viruses

Non-enveloped viruses = resistant; Enveloped = sensitive to heat, drying, detergents.

Transmission differences

Non-enveloped → fomites & gut survival; Enveloped → droplets, blood, secretions.

Older methods of viral classification

By disease, cytopathology, site of isolation, place/person discovered, or biochemical features.

Problem with early naming systems

Similar names for unrelated viruses (e.g. Hepatitis A–G); different diseases from related viruses (e.g. Herpes family).

Examples of herpesviruses & diseases

HSV-1 (cold sores), HSV-2 (genital sores), VZV (chickenpox), CMV (mononucleosis), EBV (Burkitt’s lymphoma, NPC).

Modern virus classification body

International Committee on Taxonomy of Viruses (ICTV).

Modern virus classification criteria

Presence of envelope, genome type (DNA/RNA), strand number & polarity, replication mode, size & symmetry.

Examples of DNA virus families

Adenoviridae, Herpesviridae, Poxviridae, Parvoviridae, Papillomaviridae, Hepadnaviridae.

Examples of RNA virus families

Picornaviridae, Orthomyxoviridae, Paramyxoviridae, Coronaviridae, Retroviridae, Flaviviridae, Togaviridae.

Properties of naked viruses

Stable to temperature, acids, detergents, drying; spread by fomites and droplets; released by cell lysis.

Properties of enveloped viruses

Sensitive to heat, acids, detergents; require moist environments; spread via secretions or blood; released by budding or lysis.

Why does viral classification matter?

Aids in diagnosis, epidemiology, vaccine design, and infection control.