Genomics and Virology

Viruses are tiny infectious particles that are halfway between living and nonliving organisms. They are so small (a millionth of a millimeter) that it would take hundreds to thousands of them to cover the end of a human hair. Each virus is composed of genetic material wrapped in a protein coat. Viruses that infect plants and animals also have a layer of fat molecules. Viruses cannot reproduce on their own. Instead, viruses replicate by infecting a host cell (such as humans, other animals, plants or bacteria), hijacking the host's biological machinery and turning the host cell into a virus-producing factory.

The exact reason why some viruses infect humans to cause disease and others do not remains a biological mystery. For example, humans usually die if infected by the rabies virus. But while human cells can be infected by circoviruses, they do not seem to cause disease. But circovirus infections in other mammals such as dogs and pigs can cause severe diseases.

Previously, viral vaccines contained weakened or dead viruses, with both forms being incapable of causing disease. Now, scientists have an additional tool in their toolkit, producing vaccines using a virus's genome sequence. The viral genome has the information needed to create viral proteins, the active component of the vaccine to which the immune system responds. When injected, these DNA or RNA molecules are used by the host to produce specific viral proteins, and the immune system then recognizes the viral proteins as foreign, sparking a response from multiple types of white blood cells.

One such class of white blood cells, called B cells, produces a particular type of protein called an antibody. Antibodies bind to molecules on the surface of the virus and neutralize the virus to prevent it from replicating.

Once the human body successfully produces antibodies against a virus, its arsenal is ready for defense when the immune system comes in contact with the same virus in the future. 

Yes. The human genome contains a considerable amount of DNA that previously existed in viruses. These viral sequences are remnants of past viral infections. Most of these sequences originally came from retroviruses, a type of virus that can insert one copy of its genome into the DNA of a host organism (such as a human). As the host cells make copies of its own genome, it copies the viral DNA as well. These sequences can pass from one generation to the next, becoming a permanent part of the human genome (like a fossil record).

At present, DNA from these retroviruses accounts for about 9% of the human genome, but most are thought to be incapable of producing new viral particles.