[Infectiology] Viruses

Taxonomy 

• Virus 

Viruses are a group of infectious agents.

 

Classification

There is currently insufficient evidence to determine a clear relationship between the known viruses of the Earth.

A viral species can be classified many ways. They are usually placed into large groups based on which nucleic acids they use for information storage and reproduction.

The Baltimore virus classification splits them into 7 groups:

1. dsDNA viruses (double-stranded DNA virus)
2. ssDNA viruses (single-stranded DNA virus)
3. dsRNA viruses
4. (+)ssRNA viruses
5. (−)ssRNA viruses
6. ssRNA-RT viruses (RNA with a DNA intermediate)
7. dsDNA-RT virus (DNA with an RNA intermediate)

Group I and II viruses can be categorised as DNA viruses.
Group VII viruses are sometimes categorised as DNA viruses.

Group III, IV, and V viruses can be categorised as RNA viruses.
Group VI viruses are sometimes categorised as RNA viruses

According to the strictest definitions, group VI viruses are not RNA viruses and group VII viruses are not DNA viruses. This is because their lifecycle involves both types of nucleic acid.

 

Biology

Viruses are also sometimes called 'living' because they store genetic information and reproduce to continue their existence, like a living organism.

However, viruses are different to these on the most fundemental level. One key feature of viruses is that they cannot metabolise or synthesise new genetic material alone. They are usually extremely small with small, minimalist genomes. They lack complex mechanisms for sensation and reactions to their environment. They lack an internal store of energy or nutrients. All of this means that they cannot exist alone. They infect and reproduce themselves in order to continue existing. They exist solely to infect and reproduce. There are viruses currently infecting all forms of life, even bacteria. 

 

Pathology

• Asymptomatic infection: Many viruses reproduce in human cells but do not cause any symptoms. 
• Acute symptomatic infections: These cause direct tissue destruction or acute inflammation. Many viral diseases cause permanent disfigurement, disability or death. 
• Chronic inflammation: The immune system may also react to viral infections and the resulting inflammation can cause more damage than the virus itself. Chronic inflammation is known to be destructive and carcinogenic (see: Chronic viral hepatitis).
• Oncogenesis: Viral infections can cause cell dysfunction and cell death. Sometimes this can lead to activation of proto-onco-genes and other carcinogenic events. Certain viral infections are associated with specific malignancies later in life. Immunodeficient patients often develop malignancies caused by viral oncogenesis (see: Kaposi sarcoma). 
• Evolution: There is plentiful evidence that human evolution has been influenced by DNA inserted into the genome by viral infections. (See: Placental evolution).
• Cytotoxicity: normally harmless viruses can cause rapid tissue destruction in immunodeficient patients (see: JC virus).

 

Origin

It is uncertain how viruses originated. There are many hypotheses:

1. They may have existed before eukaryotes, prokaryotes and archaea. They may have become parasitic and minimalistic over time. Their simplicity provides many evolutionary advantages. They are very efficient at converting resources into genetic information.
2. They may have arisen from living organisms. There are numerous types of self-replicating erroneous sequences in genomes. A self-replicating sequence of RNA or DNA containing genes for structural proteins could form an encapsulated miniature genome. This capsule could survive transmission from one host cell to another. Over time, the complexity of this system could increase until it becomes a virus.

 

See also

• Prion