The third postulate specifies "should", rather than "must", because Koch's experiments with tuberculosis and cholera showed that not all organisms exposed to an infectious agent will acquire the infection. Some individuals may avoid infection by maintaining their health for proper immune functioning, acquiring immunity from previous exposure or vaccination, or through genetic immunity, such as sickle cell trait and sickle cell disease conferring resistance to malaria.
The third postulate specifies "should", rather than "must", because Koch's experiments with tuberculosis and cholera showed that not all organisms exposed to an infectious agent will acquire the infection Some individuals may avoid infection by maintaining their health for proper immune functioni.ng, acquiring immunity from previous exposure or vaccination, or through genetic immunity, such as sickle cell trait and sickle cell disease conferring resistance to malaria.