Infections that have recently appeared in a population or whose incidence or geographic range is fast expanding or threatens to increase in the near future are known as emerging infectious diseases. In a 2007 report, the World Health Organization cautioned that infectious illnesses are arising at an unprecedented rate. About 40 infectious diseases have been found since the 1970s, including SARS, MERS, Ebola, chikungunya, avian flu, swine flu, Zika, and, most recently, COVID-19, which is caused by the SARS-CoV-2 coronavirus. People are travelling much more frequently and over much longer distances than in the past, living in more densely populated places, and coming into closer contact with wild animals, so the potential for new infectious diseases to spread quickly and trigger worldwide epidemics is a serious concern. Such diseases have little regard for national borders. The minority of organisms capable of efficient human-to-human transmission can become important public and global concerns as possible epidemic or pandemic causes. They can have a variety of economic, societal, and therapeutic consequences.