A virus is a small particle capable of infecting a cell and replicating itself using some of the cell's functionality. Whether a virus is alive is a subject of debate and definition. Because a virus cannot reproduce on its own, they are generally not considered organisms.
There are about 5,000 kinds of viruses and a million of different types. They are found in almost all ecosystems and the most abundant biological entity.
Virology is the study of viruses, a sub-specialty of microbiology.
Viruses can be transmitted in a variety of ways. Viruses in plants are spread by insects that feed on them. In animals, it is commonly spread by mosquitoes and other blood-sucking insects. Other viruses are transmitted by sneezing, coughing, hand-to-mouth, by physical contact or sexual contact.