Novel viruses appear all the time. They may may cause illness in a few people, but they do not have the 1918 influenza virus's ability to go on and infect large numbers. The 1957 Asian flu and the 1968 Hong Kong flu were caused by novel viruses, and reached epidemic levels in several countries, but never reached pandemic proportions. A novel virus of what at the time was called "swine flu" appeared in 1976 at Fort Dix, NJ, USA. It turned out not to be a rapidly spreading kind, but it caused a brief pandemic panic.
Since 1994, at least 30 new viruses have appeared, including Ebola Ivory Coast, the Andes virus, hepatitis F and G, Fakeeh, Pirital, Whitewater Arroyo, the horse morbillivirus, Black Lagoon virus, Nipah and the Oscar virus.
Within the past few years, the number of "new" human diseases associated with small-mammal reservoirs has increased dramatically. Rodent-borne "tropical" diseases are a specific case in point. For example, Sin Nombre virus from South America, appeared in deer mice in the USA in 1993. Rodents also spread tularemia in Kosovo. Nipah virus jumped from bats to pigs to people in Malaysia. In 1999, the West Nile virus showed up for the first time in the western hemisphere, killing seven people in New York. The proximate carriers were crows. Crows had been dying across the region, but the virus was unknown to the Department of Agriculture and the lack of communication between vets and doctors meant that the outbreak took several weeks to diagnose.