Virus
A virus is an obligate intracellular parasite consisting of genetic material (DNA or RNA) enclosed in a protein coat called a capsid, and sometimes surrounded by a lipid envelope. Viruses cannot reproduce independently; they hijack the cellular machinery of a host cell to replicate. They cause a wide range of diseases in humans, animals, and plants, and are also essential tools in molecular biology and gene therapy research.
Comparison of Selected Human Viruses
| Virus | Nucleic Acid | Envelope | Disease Caused |
|---|---|---|---|
| Influenza A | ssRNA (−) | Yes | Influenza (flu) |
| HIV | ssRNA (retrovirus) | Yes | AIDS |
| SARS-CoV-2 | ssRNA (+) | Yes | COVID-19 |
| Adenovirus | dsDNA | No | Common cold, conjunctivitis |
| Hepatitis B | dsDNA | Yes | Hepatitis B |
Interactive Tools
Wikimedia Commons, CC BY-SA
Related Terms
Bacteria
Bacteria are single-celled prokaryotic microorganisms that lack a membrane-bound nucleus and reproduce primarily by binary fission. They are among the most abundant life forms on Earth, inhabiting nearly every environment including soil, water, and the human body. Bacteria play essential roles in nutrient cycling, decomposition, and digestion, and certain species cause infectious diseases while others are harnessed in biotechnology and food production.
Pathogen
A pathogen is any biological agent — including bacteria, viruses, fungi, parasites, or prions — that causes disease in a host organism. Pathogens cause harm by directly destroying host cells, releasing toxins, or triggering damaging immune responses. Understanding pathogen biology is the foundation of epidemiology, vaccine development, and the design of antimicrobial therapies.
Prion
A prion (proteinaceous infectious particle) is a misfolded protein that can induce abnormal folding in normal versions of the same protein, propagating a conformational change without containing any nucleic acid. Prion diseases, also called transmissible spongiform encephalopathies (TSEs), cause progressive and fatal neurodegeneration by accumulating aggregates of the misfolded PrPSc isoform in brain tissue. Notable examples include scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle, and Creutzfeldt–Jakob disease (CJD) in humans.
From Latin "virus" meaning "poison", "slime", or "venom". The term was applied to disease-causing agents in the late 19th century; Dmitri Ivanovsky (1892) and Martinus Beijerinck (1898) demonstrated that certain infectious agents passed through porcelain filters too fine for bacteria, establishing the concept of filterable viruses.