Why do you get lasting immunity in viral infections?

Answer 1

You get lasting immunity to a viral infection, at least to a particular strain of virus, as a result of the viral infection itself. Viral infections are always intracellular. So once they enter the lymphocytes, the lymphocytes have sufficient time/opportunity to decode the outer protein coat of the viruses and to create the antibodies capable of attacking the virus. Here you have very few and specific outer proteins in case of the viruses. So the antibodies are few and specific. So immunity is lasting in viral infections.

Related Information:

There is partial immunity to different strains of a particular virus. To the extent that the composition of the protein differs, cross immunity becomes less effective.

You do not get lasting immunity to most bacterial infections. In bacterial infections, bacteria are usually extracellular. So lymphocytes have less time/opportunity to decode and create the necessary antibodies to fight the infection. So immunity is partial and short lived.

Concerning other intracellular infections: In these cases, also, the immunity is lasting. Examples:

In rickettsial infections, immunity is lasting. In chlamydia infections, it is lasting.

In typhoid fever it is lasting. Although it lasts for a few years only, and may loose its effectivity over time.

In malarial fever it is not lasting. ( Here it becomes lasting only due to repeated sub-clinical infections. These sub-clinical infections occur due to repeated infection from liver cells.) But the reason is obvious. The parasites stay in red blood cells and liver cells and not in lymphocytes. So there is less exposure to lymphocytes. But eventually in chronic infections, you get some immunity. (Except in falciparum malaria. Which does not become chronic.) In case of chronic infections, there is adjustment between host and infections, that they adjust with each other, as in case of Hepatitis B infection. So immunity is there and virus keeps low virulence.

In case of herpes zoster virus, it hides in posterior root ganglion. Away from immune system. It may be changing the outer coat like influenza virus, each time the virus again invades the nerve. As in case of HIV infection, the virus becomes part of the DNA. So it is not available for immune system. But during invasion of other cells, it is exposed to immune system. So you get some immunity.

We are still learning about these mechanisms, so answers are still being sought and debated.