- Viruses are generally ultramicroscopic, typically from 20 nm to 900 nm in length. Some large viruses have been found.
- Virions are acellular and consist of a nucleic acid, DNA or RNA, but not both, surrounded by a protein capsid. There may also be a phospholipid membrane surrounding the capsid.
- Viruses are obligate intracellular parasites.
- Viruses are known to infect various types of cells found in plants, animals, fungi, protists, bacteria, and archaea. Viruses typically have limited host ranges and infect specific cell types.
- Viruses may have helical, polyhedral, or complex shapes.
- Classification of viruses is based on morphology, type of nucleic acid, host range, cell specificity, and enzymes carried within the virion.
- Like other diseases, viral diseases are classified using ICD codes.
- Many viruses target specific hosts or tissues. Some may have more than one host.
- Many viruses follow several stages to infect host cells. These stages include attachment, penetration, uncoating, biosynthesis, maturation, and release.
- Animal viruses can undergo latency, similar to lysogeny for a bacteriophage.
- Other acellular agents such as prions also cause diseases. Prions are proteinaceous infectious particles that cause transmissible spongiform encephalopathies.
- Prions are extremely resistant to chemicals, heat, and radiation.
- There are no treatments for prion infection.