The upper respiratory tract is the natural habitat for
many potential pathogens, including viruses, mycoplasmas,
chlamydias, and many other bacteria. The
commensal flora may have a favorable competitive
effect for their host by outnumbering pathogenic
agents. There is no distinct division between commensals
and potentially pathogenic microorganisms. Different
studies categorize the same microorganism
as either commensal or potentially pathogenic. For
example, Mycoplasma flocculare, Mycoplasma hyorhinis,
and Haemophilus parasuis belong to a group of microorganisms
that can regularly be isolated in the upper
respiratory tract of healthy pigs.