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.
Viral diseases are nature's population control mechanism. In nature, when populations soar and become densely packed, viral diseases tend to break out; then the population drops. The human species has quadrupled to 6 million in one century and is heading for 9 or 10 million by 2050.
The global loss of biological diversity affects the wellbeing of both animal and people. Four out of five conditions in humans are environmentally related, so we have already a connection. And the connection is health. For example, habitat damage increases stress in living things, causing greater susceptibility to disease across species and geographic boundaries. The loss of species, the degradation of ecological processes and the contamination of the web of life are working in concert to diminish human and environmental health on this planet.