Propagated or progressive epidemics
Propagated or progressive epidemics occur when the infection spreads from person to person. The infectious agents causing the disease pass from one host to another, either directly from person to person (e.g. via hand shaking or kissing), or indirectly via vectors (e.g. mosquitoes in the case of malaria), or in water, food or another medium. The distribution of malaria cases is a good example of a propagated epidemic, because increased numbers of malaria cases occur again and again at different times. Propagated epidemics last longer than the common source outbreaks described above. This is because malaria will continue to spread in the community, as long as mosquitoes are present in the environment and there are people who carry the parasite.
Can you think of any epidemic-prone diseases that spread quickly in overcrowded conditions where there is poor sanitation and personal hygiene?
You may have thought of typhoid fever, cholera, shigellosis (bacterial dysentery), louse-borne relapsing fever and typhus. (Think back to Study Sessions 33 and 36.)