Hookworm is an intestinal parasite that infects humans mainly in tropical and subtropical climates. Hookworm is estimated to infect one fifth of the world's population, about one billion people. Symptoms usually include mild diarrhea or cramps, but heavy infections can cause more serious health problems in children and the malnourished. Infection is caused by direct contact with infected soil, and it cannot be passed directly from person to person due to a complex life cycle that requires warm, moist and shaded soil for larval stage of development. Inside humans, after traveling through the body to the intestines, the hookworm grows to be a half-inch-long worm that sucks blood. The adult then lays thousands of eggs that can then infect soil again.