It depends on what kind of relationship you have with your boss. My gut feel is explain to them that you are physically ill and you know you aren't performing well because of it.
People may say they understand but if you tell them you have parasites
they are going to be subconsciously creeped out. They will treat you differently. I just don't think telling them is in your best interest
I think everyone should get a grip and realize parasites
are a real problem BUT people are stupid.
Same reason they treat disabled people with hidden disdain.
If they insist on knowing make up something like IBS or just tell them you would prefer to keep your private life private.
I've tried explaining the parasite
problem to friends and family but they are all in denial because they are sure if they had them their Doctor would know. They just assume I am a weirdo. The truth is people just don't want to know things they can't deal with. Save yourself the frustration.