I've never heard of this.
If it's true, I'm pretty sure that it must be due to virusses fuctioning "Best" at certain temperatures and "less well" at other temps.
So, changing the body temperature may render them "Less useful", meaning that they have a harder time infecting you.
The body temperature rises to combat virusses (and bacterial infections as well).
When the body gets too hot you need to cool it down. That's why you take aspirin and the like. Or cool showers or a flannel with cold water to the head.
Maybe that's what you're referring to? But then you're already infected!