Even if water looks pristine or is running, or if you melt it from snow, it can be contaminated, as can swimming pools, even if chlorinated. And, rarely, when their filter systems break down, even municipal water systems in the U.S. may become contaminated.
Pets can also become infected and can spread the parasite to humans.
It takes that long for the organism to grow in the intestinal tract and cause symptoms. So you may have trouble pinpointing exactly when you became infected. These symptoms can continue for up to four months or longer. But most infected people never develop symptoms, or get over the infection in one to four weeks without treatment. On the other hand, some people develop a chronic infection, with or without symptoms.