Viruses are infectious agents of nucleic acid contained in a capsid. Some also have an external envelope which aids host entry. Their phenomenal replication rate relies on the infection of living host cells to utilise the host machinery.
Viruses have been found inside every life form but vary in their toxicity to the host cell. However, the effects of the viral infection can vary depending on the host organism. For example, the same virus may cause a fatal feline disease but not cause symptoms in humans. Some viruses confer protective properties to the host, such as those found naturally as part of the gut microbiome to prevent Escherichia coli colonisation.
Viral replication and dispersion can lead to cell death, seen in various human conditions, including AIDS, COVID-19 and smallpox. Research is uncovering how viruses infect and proliferate in human cells. This knowledge may create new methods to prevent human viral replication, thus preventing these illnesses.