Why is killing its host usually disadvantageous for a parasite?

Sometimes the parasite might live off of the host.

It leaves the parasite stranded in the host making it tough to survive!

But also nature can take it's natural course and solve the problem by

- - You Guessed It - - Mutation, adapting to the problem. By mutation

the parasite may be able to make itself air-borne, spreading from host

to host!