In addition to chemicals and radiation, another source of mutation is viruses. Viruses are very small 'organisms' that can infect the cells of other animals or plants. Humans are susceptible to a large number of different viruses. Viruses are not the same as bacteria although both can cause human disease. Treatments that cure bacterial infections are not useful in the treatment of viral infection. Some examples of viruses include the agent that causes the flu (influenza virus) and the causative agent of AIDS, the human immunodeficiency virus (HIV).
Viruses can disrupt cell behavior in several different ways.
They can directly cause DNA damage (mutations) by inserting their genomes into the DNA of the host cell. The integration can disrupt important regulatory genes.
The viruses may contain their own genes that disrupt the regulation of the cell. This process may be beneficial to the virus if it allows for rapid production of progeny but can be seriously detrimental to the host.
Some viruses actually carry altered versions of genes that they have picked up from previous host cells. These altered genes no longer function properly, and when they are inserted into a new host cell, they cause disregulation and can lead to cancerous growth.
Through their mutagenic activity or their effects on cell behavior, viruses play a significant role in the development of particular cancers in many different animals, including humans.
Viruses have also been a major target of scientific investigation with respect to cancer. Some of the earliest work on the identification of oncogenes and tumor suppressors utilized viruses.(1)
Viruses can be divided into two rough categories, those that have DNA as their genetic material and those that have RNA as their genetic material. Both kinds of virus have been found to be associated with cancers of different types. The viruses known to cause human cancers are: