[couradical]

Sure it can - think of smallpox: the first smallpox "vaccine" was actually inoculation with cowpox - a similar, but less virulent virus. The provoked immune response rendered a smallpox infection less lethal, and provided a measure of protection.

- http://en.wikipedia.org/wiki/Smallpox_vaccine

[ars]

That's quite different, we are speaking of have two pathogens compete, not about triggering an immune response.

If you took someone who is already infected with smallpox and gave them cowpox it would not help.

You confused in what order the infections occur.