Intestinal Parasites

Parasites suck... literally! They always cause disease, thus they are termed parasitic as opposed to commiserate organisms. Parasites are spread in unclean conditions (which are not typically found in the western world) so, unless you have been camping or in close proximity with a person or animal harboring a parasite, your IBS symptoms are unlikely being caused by a parasite. Intestinal parasites cause severe illness and so if you are ill, always see your doctor, a stool test will rule this out or if the history is appropriate, a medication will be called out.
As a side note - there are other types of parasites other than intestinal, and again these are rare in the US, however, if you are pregnant you do need to know that cats can carry a parasite called toxoplasmosis which causes birth defects... continue to wash your hands before you eat, wear shoes outdoors, cook your food properly and relax... it is unlikely you have a monster with you.