Despite being so small they can't be seen with the naked eye, pathogens that cause human disease have greatly affected the way humans live for centuries.
Here we explore our past and present struggles with four of the most significant infectious diseases human beings have faced, and some of the progress we've made in prevention and treatment.
Edward Jenner deliberately infected a young boy with cowpox and then smallpox. But his method wasn't as mad as is often made out.
The positive impact of the polio eradication initiatives on the continent can be felt across the health sector in other health programmes.
It's been one year since the last polio case was reported in Africa. If the continent keeps this up, it could be declared polio free by 2018.