A simple example is a person with antibodies being sneezed on with infectious agents. That person could coincidentally sneeze on another person who lacks antib

A simple example is a person with antibodies being sneezed on with infectious agents. That person could coincidentally sneeze on another person who lacks antibodies, but wouldn't worsen themselves since the contagion in the original sneeze would get met with prepared antibodies killing off the "population" faster than it could replicate.