Fight to the finish
Understanding host–pathogen interactions
An infection occurs when a population of pathogens (the disease-causing agents) grow in or on an organism (the host), affecting its function. The host population might be made up of people, animals, plants or even bacteria. Some pathogens infect more than one host – like the flu viruses that infect birds as well as humans, and the tuberculosis-causing bacteria that infect cows and badgers.
At the individual level of the host, an infection is a battle between its defences – its immune system – and the strategies that the pathogen population deploys to bypass these defences. A healthy human immune system will fight off the pathogen and develop antibodies that can recognise it in the future. At the population level this creates an arms race between the pathogen population and the host population, in which the pathogen evolves new strategies to overcome the hosts’ defences. In human populations these defences involve drugs and vaccines as well as naturally developed immunity.
Viruses can only live and reproduce inside their hosts’ cells. Any flu virus that leaves your body via a sneeze will only survive on a door handle or a kitchen surface for a matter of hours. This is why viral epidemics can only sustain themselves if enough new, susceptible members of the population are available to spread the infection. The success of measles and cervical cancer vaccines depends on critical thresholds of vaccination being reached within a population, thereby preventing the viruses that cause these diseases from moving easily between one host and another. This concept is known as ‘herd immunity’.Lead image:
Illustration © Glen McBeth