I always see this in the summertime - folks swimming in lakes, ponds, rivers, streams and even chlorinated pools - both here and abroad - who will come down with what they THINK is the stomach "flu" or a stomach "bug" - when in fact they have just picked up a parasite infection.
The difference in the two is this - a small bout of "bug" or "flu" will clear up in a few days and leave no lasting effects - whereas a PARASITE infection will continu causing health problems for days - weeks - months even years and decades!
The health problems just build and build. This is when you KNOW that the stomach and digestive problems are parasite related.