A virus is an infectious microbe that resides in a gray area between the living and nonliving worlds. Unlike bacteria and other microorganisms, viruses cannot grow or reproduce on their own. Instead, they parasitize cells and rely on a host cell’s replication machinery to make copies of themselves. Viruses are composed of nucleic acid (a set of genetic instructions, either DNA or RNA) encased in a protein coating called a capsid. They are shaped like little soccer balls, cylinders or tubes (helixes). Depending on the type of virus, they can be single-stranded or double-stranded, linear or circular, and containing one or two sets of genes.
Viruses infect a wide variety of plants, animals and bacteria, each targeting specific types of hosts. Human viruses cause many diseases, including the common cold, influenza (flu), measles, rubella, mumps, shingles, hepatitis and HIV.
Most viruses get into the body through mucous membranes, such as those in the eyes, nose and mouth. Some viruses may also enter through the skin or from contact with an infected animal, mosquito or tick. Others get in by breathing droplets or particles that contain the virus from someone who has it. They can also spread through vaginal, oral or anal sex or by touching surfaces that have the virus on them.
Some viruses have a lipid membrane that surrounds their protein coat, which is called an envelope. These are called enveloped viruses, while others do not have an envelope and are referred to as naked viruses. Viruses are classified by the type of cell in which they replicate, as well as their structural proteins and whether they have an envelope or not. There are over 3,600 different virus species. Traditionally, they have been grouped into “species” according to the Linnaean system of classification, but new methods are being used that categorize them based on nucleic acid homology.