Viruses are much smaller than fungi or bacteria, and they must invade a living cell to reproduce, or replicate. The virus attaches to a cell, enters it, and releases its DNA or RNA inside the cell. The DNA or RNA of the virus is its genetic information, which takes control of the cell and forces it to replicate the virus. The infected cell usually dies because the virus keeps it from performing its normal functions. Before it dies, however, the cell has already released new viruses, which go on to infect other cells.
Some viruses do not kill the cells they infect, but instead change the cells’ functions. Sometimes, the infected cell loses control over normal cell division and cancer cells are produced. Some viruses that do not kill the cells they infect leave their genetic material in the host cell, where it can remain inactive for a long time. This is called a latent infection. When the cell is disturbed, the virus may begin to replicate and cause disease.
Viruses are transmitted in a variety of ways, depending on the body system affected. For example, common viruses of the respiratory tract are usually inhaled, and viruses of the digestive tract are often swallowed. Other viral infections are transmitted by the bites of insects and other parasites (such as mosquitoes and ticks). Most viruses infect only one or a few species. For example, canine parainfluenza virus does not infect people. Feline leukemia virus does not infect dogs. Rabies is an exception and can affect any mammal.
The body has a number of defenses against viruses. Physical barriers, such as the skin, discourage easy entry. Infected cells also make substances that can help non-infected cells resist infection by many viruses.
When a virus enters the body, it triggers the body’s immune defenses. These defenses begin with white blood cells, such as lymphocytes, which attack and destroy the virus or the cells it has infected. If the body survives the viral infection, the lymphocytes “remember” the invader and can respond more quickly and effectively to a later infection with the same virus. This is the basis of adaptive immunity. Adaptive immunity can also be produced by vaccination. Drugs that fight viral infections are called antiviral drugs. Antiviral drugs work by interfering with viral replication. Because viruses are tiny and replicate inside cells using the cells’ own mechanisms, there are only a limited number of metabolic functions that antiviral drugs can target. In contrast, bacteria are larger organisms, commonly reproduce by themselves outside of cells, and have many metabolic functions against which antibiotics can be directed. Therefore, antiviral drugs are much more difficult to develop than antibiotics. In addition, viruses can develop resistance to antiviral drugs. The antiviral drugs themselves can also be toxic to animal and human cells.
Antibiotics are not effective against viral infections, but if a pet has a bacterial infection in addition to a viral infection, an antibiotic is generally needed.