The basic answer is that humans are an evolved system, and any simple unambiguously helpful change in the context of the environment of early humans spread fair

The basic answer is that humans are an evolved system, and any simple unambiguously helpful change in the context of the environment of early humans spread fairly quickly throughout the entire population. If there was an infectious disease that improved sense of smell through any simple mechanism, it's almost certain that either some human somewhere had a mutation that did the same thing, or that "infectious disease" will become a symbiote (see gut bacteria for example). So the basic answer is "you don't see them because you'd only notice them in the time between first being seen and being present in everyone in the population, which is a very short time compared to the time before they first occurred and the time after they reach 100% of the population".