Col. Potter says schistosomiasis comes only "from wading waist-high in the River Nile," but while its causative worms are not found in the U.S., it occurs due to contaminated water elsewhere around the world, including southeast Asia.
It makes no sense for B.J. and Hawkeye to quarantine Margaret's tent when there was a VIP tent that was almost always empty and could have easily worked for a rendezvous.