organisms including bacteria and viruses express genes at varying levels, leading to diversity in the phenotypes of the organisms in these populations. Depending on the genes, certain bacteria may have an advantage over other due to a higher or in some cases, a lower level of expression of that gene. Hence, in times of stress, the majority of a bacterial population will be killed but the minority of cells expressing the fluctuating levels of the gene will survive.....meaning cleaners and the like will never completely wipe out a population.
and someone up there said some people are immune to cancer....not true. Everyone is susceptible to cancer. Cancer is caused by mutations in DNA that switch off tumour supresser genes. These mutations are caused by things like UV radiation, smoking, exposure to carcinogenic chemical etc.