I have a friend who's a parasitologist (not M.D., but Ph.D.). Here's her response (before your post about S. Padre Island):
Wow..... Ok, well without knowing what he and his daughters did together prior to the one leaving for camp, it is really hard to say. In most cases if people are outdoors and get 'sick' at the same time, often it can be Giardia, Cryptosporidium and the like (at least in North America........). Local water sources can also get contaminated with Crypto, but that can be discovered if you ask, even just asking their local water source. It happens more often on cruise ships, hotels, public place etc. that homes. But if this guy hunts with his girls, Giardia is a very likely culprit. But, with that, you can get a fecal test (several when it comes to Giardia, sneaky bastards....). I would expect if he had any inkling that it was parasites, he should have asked for a test, or even find a doctor that actually knows something about parasites, like a trop med person - it will be expensive no doubt, but they are used to worms, Giardia, malaria, Crypto, amoeba (this is a case where it often ends with dysentary, while Giardia does not) so they are skilled in parasite finding, whereas MOST all of US doctors are NOT......
If they have been officially tested for lactose intolerance, not clear if so, then maybe it is. If he does not believe genetics, he has only to ask his family......
And yes, there are many parasites you can get without fever and bloody diarrhea!!! But that description usually most fits amoeba and heavy Cryptosporidium infections, again, in North America. Giardia can, but they like to maintain a more functional host!!
Well, that is all I can think of without digging around!
