As a general rule, a viral infection kills its host cell, but some cells escape destruction by harboring the virus in some form. These so called persistent infections can last from a few weeks to years, even for the life of the host. As an example, the hepatitis B virus produces a chronic progressive infection of the liver that may lead to cirrhosis and cancer. Many viruses remain in a latent state, meaning that they remain inactive or nonproductive over long periods. Examples of this are herpes simplex viruses (cold sores and genital herpes) and herpes zoster virus (chickenpox and shingles). Both viruses go into latency in nerve cells and later emerge under the influence of various stimuli to cause recurrent symptoms. Some animal viruses integrate their DNA into the DNA of the host cell, sometimes leading to cancer. Such viruses are termed oncogenic, and their effect on the cell is called transformation. Transformed cells generally have an increased rate of growth, alterations in chromosomes, changes in the cell’s surface molecules, and the capacity to divide for an indefinite period. Examples include DNA viruses like papillomavirus, which is associated with cervical cancer, and herpesviruses like Epstein-Barr virus, which is a cofactor in Burkitt’s lymphoma, a cancer affecting one type of white blood cell.
Cancer is not the only outcome of viral integration. For millions of years, humans—or the forbears of the species—have endured viral infection, and we harbor evidence of ancient viral infection in our DNA. As much as 8% of the DNA in the human genome (the total of all the DNA in a human being) is viral in nature, an astonishing amount when you consider that less than 2% of human DNA codes for proteins.
Much of this DNA is in the form of endogenous retroviruses, which have converted their RNA to DNA, inserted the DNA into a site in one of our 46 chromosomes, and then become dormant. If this occurred in cells that gave rise to egg or sperm cells, the virus could be transmitted virtually unchanged for hundreds of generations.
Virologists do not agree on the extent to which ancient viruses interact with modern humans, but the evidence is compelling that we owe certain aspects of our biology to ancient viral infection. One type of endogenous retrovirus has been shown to be intimately involved in forming the human placenta, leading microbiologists to conclude that some viruses are an essential factor in evolution and development. Other retroviruses may be involved in diseases such as prostate cancer and chronic fatigue syndrome, while strong evidence links certain viruses to human obesity. Several studies with animals revealed that chickens and mice infected with a human adenovirus called Ad-36 had larger fat deposits and were heavier than uninfected animals.
