Completar frases virusVersión en línea biotechnology por ATTILIO RAPACCIOLI 1 others case range abundant harmless outbreaks Other viruses viruses Viruses are by far the most biological entities on Earth and they outnumber all the put together . They infect all types of cellular life including animals , plants , bacteria and fungi . However , different types of can infect only a limited range of hosts and many are species - specific . Some , such as smallpox virus for example , can infect only one species in this humans , and are said to have a narrow host range . viruses , such as rabies virus , can infect different species of mammals and are said to have a broad range . The that infect plants are harmless to animals , and most viruses that infect other animals are to humans . The host of some bacteriophages is limited to a single strain of bacteria and they can be used to trace the source of of infections by a method called phage typing .