In this story an epidemic or pandemic occurs.
Epidemic occurs when new cases of a certain disease, in a given human population, and during a given period, substantially exceed what is expected based on recent experience. Generally, an epidemic occurs when host immunity to a parasite population is suddenly reduced. One of the reasons for this may be the emergence of new species of viruses, bacteria, etc. An epidemic may be restricted to one location; however, if it spreads to other countries or continents and affects a substantial number of people, it may be termed a pandemic.
In some fictional stories, the cause of the epidemic or pandemic can be the use of biological weapons. This may lead to a zombie apocalypse, or other interesting effects.