but wouldn't a dog who is less stressed and older and more stable be
better able to fight it off?

*Possibly*, but unlikely. They are probably still hosts but do not exhibit
symptoms as prominently - probably co-exist with the critters. Fecals are
only 50% accurate. We advise anyone with a pet in a household with young
children or folks that are immuncompromised to do strategic deworming as a
preventive.

A good site to check out is the Companion Animal Parasite Council at
www.petsandparasites.org