Viruses do not belong in any of the five traditional kingdoms of Monera, Protista, Fungi, Plantae and Animalia, but some biologists say they belong in their own kingdom. Viruses lack cells, one criterion used to place entities into the traditional kingdoms.
In addition to lacking cells, which are the fundamental units of life, viruses fail to perform other functions that living organisms placed in traditional kingdoms do, such as eating and producing waste. The main function of a virus is to replicate, which it accomplishes through the genetic material of either DNA or RNA, depending on the virus. One common example of a virus is the chickenpox virus.