Viruses are germs that infect and make people sick. They are the smallest organisms that can cause disease. They are made of genetic material in a “carrying case” called a capsid, which protects them until they can infect host cells. Unlike most other organisms, viruses do not have the machinery (enzymes) that they need to perform all of the processes needed for life. Instead, they take over the host cell’s equipment to make more viruses and replicate.
Vaccines and antiviral drugs are used to prevent or treat viral infections. A person can also get a virus by breathing in droplets of virus-infected air or through contaminated food, water or blood. Most viruses enter the body through mucous membranes, including the eyes, nose, mouth, throat, penis and genitals. Viruses may also be spread by bites from insects or animals.
There are different kinds of viruses that infect bacteria, plants and animals, each infecting only certain types of cells. The viruses differ from each other in the way they infect their hosts, how they infect those cells, and the diseases they cause.
Most viruses have a protein on their surface that can bind to the receptor proteins on the surface of host cells. Once bound, the virus can invade the cell by entering through the cellular membrane and hijacking the host’s enzymes to start producing more viruses. Once the number of viruses in a host cell reaches a critical mass, it explodes (lyses) and releases the virions into the surrounding cells. Viruses can also remain in a dormant or silent phase until certain triggers, like stress, chemical signals or temperature changes, activate them to begin their reproduction cycle again.