As we've mentioned before, infectious diseases often cause changes in body odor. But immunizations, interestingly, can have similar effects. In an animal study published in 2014 in the journal Physiology and Behavior, researchers from the Monell Chemical Senses Center and the U.S. Department of Agriculture demonstrated that immunization can trigger a distinct change in scent. Scientists believe that humans and other animals may give off "immune-activated odors" as a way of signaling to other members of their species that they have become infected with diseases. (This could also explain why certain diseases have specific odors).
Researchers found a pathway between immune activation and changes in body odor compounds, and believe that eventually, it may be possible for doctors to use odor to "eavesdrop" upon the immune system and make noninvasive diagnoses [source: ScienceDaily].