A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.
There are millions types of viruses, ~ 5,000 virus species have been described in detail. Viruses are found in almost every ecosystem and are the most abundant type of biological entity. The origins of viruses in the evolutionary history of life are unclear.
While not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles, virions, which consist of the genetic material made from either DNA or RNA, a protein coat, and in some cases an outer coat of lipids. Virion is about one one-hundredth the size of a bacterium.
A sharp distinction between humans and “animals” is essential if we are to bend them to our will, make them work for us, wear them, eat them–without any disquieting tinges of guilt or regret. It is unseemly of us, who often behave so unfeeling toward other animals, to contend that only humans can suffer. The behavior of other animals renders such pretensions specious. They are just too much like us.