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.
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.