Hii!
Rog pratirodhak kshamta means the immunity in once body. And the concept behind the question you asked is the technique of vaccination. There are many diseases for which once a vaccine is given, it never occurs agin such as diphtheria, measles, tuberculosis, hepatitis B, meningitis, rubella. This is why infants are given certain vaccines at certain age.
Certain diseases have been completely eradicated such as  polio, small pox etc.,
But, for some diseases time to time immunisation is required such as tetanus.
And, this also varies from person to person. This is science, exceptions are always possible.
Hope this helps.
Good luck!