There are literally thousands of varieties of the common cold virus. You are probably immune to each one you have had in recent years, but that leaves a lot of infection still out there.
There are several strains of ebola, it would be interesting to know if having one strain confers immunity to the others as well. If so that would leave hope for an attenuated vaccine (like the smallpox vaccine) for ebola....
I was also curious if it could mutate enough that even people who fought it off the first time would have to do it again after it evolved a while, even if it was the same base strain they were exposed to earlier.