Viruses are familiar from the common diseases they cause: colds and flu, for instance. But what are they, and how do they cause sickness?
A virus is a tiny, infectious particle made up of an outer layer called a capsid that's wrapped around a strand of DNA or RNA. DNA and RNA are chains of genetic material that contains instructions for the virus to reproduce. Some viruses also have a lipid (fatty) membrane surrounding their outer layer. Some have enzymes, a type of chemical that helps them reproduce inside a cell.
A virus can be thousands of times smaller than a bacterium, small enough to pass through most filters made to trap bacteria. Viruses are found everywhere in nature, even in harsh environments like deserts and polar seas, and thousands of feet underground. They also make up the bulk of organic matter in the sea. They infect plants, animals, bacteria and humans.
Scientists estimate that there are millions of types of viruses, most not yet discovered. So far, each type of virus that has been discovered has its own unique genetic makeup. This means that viruses may represent the largest reservoir of genetic material on earth. Viruses also may create new combinations of genetic material as they reproduce. This means that they create new or mutant versions of themselves.
Unlike living cells, viruses cannot on their own carry out the biochemical processes needed to reproduce. They must be inside a living cell to function and produce more viruses. But viruses are very specific about what type of cell they need.
How a virus infects a cell
Some viruses can remain outside a cell for a long time. Others can survive only in certain conditions. The virus's capsid protects the virus when it is outside a cell. Some viruses have capsids that are resilient and can withstand different environmental conditions. Others are fragile. The capsid also determines the path by which the virus enters a living organism. It also identifies the type of cell in the body that will host the virus.
Viruses usually infect only one type of cell. Once a virus finds the appropriate cell, it attaches itself to the cell wall. The virus then either enters the cell, or injects its genetic material (and enzymes, if it carries them) into the cell. Once inside the cell, the viral DNA or RNA and viral enzymes use the host cell’s own machinery to produce copies of the virus. These newly created copies leave their host cell by exploding out of it -- killing the host cell -- or breaking through the cell wall in a process called budding. The new viruses then find and infect other host cells.
Viruses can stay in the body area or organ they first infect, or they can spread. Viruses that cause hepatitis, for example, infect the liver and remain there. The measles virus and varicella-zoster virus enter through the respiratory tract and spread to lymph nodes, skin and other organs. Viral infections can damage body tissues in several ways. They can interfere with the normal processes of the host cell, kill the host cell by exploding out of it or trigger the immune system's response.
In people with a healthy immune system, most common disease viruses produce infections that last from seven to 14 days. Some viruses, however, can cause chronic infections. Others lie undetected in the body and cause symptoms at a later time, called a latent infection. In a chronic infection, the virus reproduces and causes effects for an extended time, perhaps for a person's entire life. Hepatitis B and C viruses cause a chronic infection. In a latent viral infection, the virus's DNA or RNA rests harmlessly in the host cells and does not reproduce. If the virus is eventually activated, it begins to reproduce and damage body tissues. Varicella viruses are examples of viruses that cause latent infections. The varicella-zoster virus remains in the body after causing the initial infection known as chicken pox. After the initial infection, it enters the nerves and travels to base of the spine, where it remains dormant, not reproducing and not causing tissue damage. If it is re-activated, it travels through nerves to the skin, where it causes the blister-like lesions of shingles. The lesions appear along the route that the affected nerve follows underneath the skin. The virus then returns to its dormant state.