Feces are mostly made of water (about 75%). The rest is made of dead bacteria that helped us digest our food, living bacteria, protein, undigested food residue (known as fiber), waste material from food, cellular linings, fats, salts, and substances released from the intestines (such as mucus) and the liver.
This warning was most likely issued to prevent people from bringing SARS to the USA when they come back from visiting the countries. So the answer is pretty much to stop the spread
