We now know that viruses are found as parastics in all types of organisms. They can reproduce only within cells. Many diseases of humans are known to be caused by viruses; some diseases of agriculturally important animals and plants are also known to be caused by viruses. Viruses infect fungi, bacteria and protists as well.
Advances in molecular biology techniques in the 1980s and 1990s have led to the discovery of new human viruses. Hepatitis C and Pestivirus which causes severe pediatric diarrhea were discovered in the 1989. Public health officers are concerned about health risks posed by thses new viruses and they are also concerned that the ease of world travel and changing enviroment will spread viruses to new areas. The 1993 outbreak of a Hantavirus infection in the southwestern U.S.A illustrates the potential for new infections to emerge at any time. Hantavirus diseases is an acute, often fatal, respiratory diseases.Although the virus had previously been isolated from rodents, the disease had not been reported in humans in the U.S.A.