It weakens the community. I was thinking of selection on a community (or higher) level, e.g. a community that has different practices will be stronger and either outlast or conquer the community with dangerous traditions.
It takes catastrophic levels of disease (think: Smallpox introduction into the entirely susceptible North and South American populations) to sufficiently weaken a culture to the point of conquerability.
Consider the number of European nations in the middle ages fully capable of going to war with high endemic disease levels, or indeed lasting on the field while there were ongoing epidemics in their armies.
