Viruses are microscopic entities that consist of genetic material (DNA or RNA) wrapped in a protective coat called a capsid, made of protein. Some viruses also have a lipid envelope, derived from the host cell, around them. The capsid may be spherical, or it may be helical, with spikes that are used to bind to and enter host cells. Viruses can only reproduce in a living cell.
When a virus comes into contact with a host cell, it attaches to unique receptor sites on the surface of that cell. The virus then hijacks the host cell’s machinery, diverting the cell’s metabolism toward making more viruses. This process is known as the lytic cycle. Eventually the host cell becomes so overwhelmed by virus production that it breaks apart, or lyses. The host cell then releases hundreds of thousands, or millions, of viral particles, which can infect other cells, thereby spreading the virus.
Some viruses are highly selective in the types of cells they infect, focusing on bacteria, plants, or animals, but most viruses attack a wide range of host cells. Some viruses attack only specific organs, such as the liver or lungs. Others target particular tissues, such as the lining of the nose.
Viruses are not considered to be “alive” in the same sense that plants or animals are alive, because they do not produce adenosine triphosphate, which is needed for cellular energy production. Instead, they are better viewed as parasites, in a similar way that a bean seed is a parasite of the plant on which it will grow.