Viruses are much smaller than bacteria or fungi and must infect living cells to reproduce.
A virus attaches to a cell, enters it, and releases its genetic material (DNA or RNA). This genetic material takes over the cell’s internal processes and forces it to make more viruses. The infected cell usually dies but releases many new viruses first, which then infect other cells.
Some viruses do not kill infected cells but change how these cells function. They might cause cells to divide uncontrollably, leading to cancer. Some viruses leave their genetic material in host cells, remaining in the body in an inactive form. Under certain conditions, these viruses can reactivate and cause disease. This is called a latent infection.
Viruses spread in different ways:
Respiratory viruses are inhaled.
Digestive viruses are swallowed.
Some viruses are spread by insect bites (mosquitoes, ticks, etc.).
Most viruses infect only one or a few species. Canine parainfluenza infects dogs but not people. Feline leukemia virus infects cats but not dogs. Rabies is a major exception because it can infect both animals and people.
Physical barriers like skin help block infection by viruses. Infected cells can produce substances that help nearby cells resist infection. The immune system responds to infection by using white blood cells to attack viruses and infected cells. After a viral infection, some of these white blood cells remember the virus and can respond faster to fight off infection if it returns. This process, called adaptive immunity, is similar to what happens in vaccination.
Antiviral drugs prevent viruses from reproducing, but viruses are harder to target than bacteria because they take over and use the host cell’s internal processes. Therefore, there are fewer points where drugs can safely act to kill the virus without also harming the cell. Some antiviral medications can prevent the virus from becoming part of the host’s DNA. Viruses can become resistant to antiviral drugs, and some antiviral drugs can be toxic to cells.
Antibiotics and antifungal drugs do not work against viruses. However, if a bacterial infection is present along with a viral infection, antibiotics might be needed to treat the bacterial part of the illness.
