Human beings have, for good reason, spent a considerable amount of their history figuring out ways to hide their excrement. In fact, several good reasons, reasons so small you need high-powered microscopes to see them. Behold, for instance, shigellae. Resembling a tiny ice-cream sprinkle, this bacterium is among scores of microscopic critters that may lurk in human waste. If able to enter your body, usually through the mouth, it heads directly for your large intestine and gets busy. Your blood pressure decreases as you rapidly dehydrate, and blood flow to the brain slows to a trickle. Convulsions may ensue, but that's usually as bad as it gets.
Not so for Asiatic cholera, another infectious disease easily spread by fecal contamination. It's caused by the bacterium Vibrio cholerae, a little curved bugger 10,000 times smaller than a Tic Tac. It travels in huge armies that set up camp in your intestines, turning whatever was there into a liquid resembling barley soup. If you become its unwitting host, it will show its appreciation by inducing vomiting, shriveling your skin, and violently cramping up your limbs. You will become so dehydrated you'll stop urinating (but you won't have much to urinate anyway, because your body won't accept fluids). If untreated you will probably lapse into a coma and die.
Unfortunately, the best available water-quality test, which measures coliform bacteria, has limited value because those bacteria die rapidly in salt water. The test also can't detect other sewage-borne bacteria, viruses, and parasites that may linger well after coliform has passed away. Recent research, for instance, concludes that the virus suspected of causing AIDS may be able to survive in sewage for up to three days.
Then there is the usual array of pathogenic organisms that can cause A among other ills A fever, rashes, diarrhea, acute conjunctivitis, respiratory infections, herpes, gastroenteritis, typhoid fever, nephritis, giardiasis, pneumonia, hepatitis, paralysis, and meningitis. In fact, scientists have found more than 100 different viruses in sewage.
While many microbial delights at least have the decency to announce their arrival early and loudly, you couldn't say as much for the beef tapeworm, one of several multicellular, feces-related parasites that may invade your body. This cestode will head directly for your intestine and make itself right at home. It can grow to 25 feet and stay around for years, but in most cases there are few symptoms except for the occasional bout of minor diarrhea. You will probably never even know you have a new buddy.