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.
Humans play host to many little passengers. Right now, you’re incubating, shedding or have already been colonised by viral, bacterial, parasitic or fungal microorganisms - perhaps even all of them.
With so many microbes capable of hijacking and destroying us, how are we, as a species, still enduring?
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.