Koch's postulates are:
- Microorganism is found in all the infected organisms, but should not be present in healthy organisms.
- Microorganism can be isolated from an infected organism and cultured in lab.
- Lab cultured organism should cause disease in healthy person when injected.
- Microorganism needs to be isolated from the infected organism in step 3, the infected organism and the isolated microorganism should be checked with the case in step 1 and 2.
Some individuals contain alternate versions of specific genes (alleles) that make them resistant to particular diseases. for example, people with certain globin alleles are resistant to malaria, and some people cannot be infected with HIV because, genetically, they lack the co-receptors that the virus needs to enter cells. This will lead to the failure of Koch's 3rd postulate.