hi sakia,
Im not sure which book you're using but in Ananthanarayan and Panicker's txtbk of micro it says that premunition is a type of natural active immunity that only lasts as long as the original infection remains active, once the patient is cured they become susceptible again (p 76). Like the Professor said it happens in syphilis. Another definition i found online was: "resistance to infection by the same or closely related pathogen established after an acute infection has become chronic, and lasting as long as the infecting organisms are in the body."(http://medical-dictionary.thefreedictionary.com/premunition)
I'm not sure of the exact mechanism of how it works though.
Hope this helps and if anyone has anything else please post it because i couldn't find much thanks

That's wonderful Nadia. Please continue to use this forum for Micro Discussions.

i also found out that it can be resistance to a particular infection due to previous presence of the causative agent.but i aslo dont understand how it can be used in malaria.anyone with answers can help out.

I don't think it is "used" any where. It is just that till such time previous infection is in place new infection by the same organism does not take place...some consolation!!