Trichuris trichiura is the whipworm species that primarily infects humans. Greater incidence of infection is present in regions with heavy rainfall, a subtropical climate, or highly polluted soil. Infection rates are also highest in children, who may play in soil contaminated with eggs of the parasite and do not always understand the necessity of good hygiene. Some human hosts do not suffer any symptoms related to whipworm infection, but if the parasite burden is high enough diarrhea, dysentery, anemia, pallor, loss of appetite, and other problems may arise. In cases involving large infestations in children, physical and mental retardation can occur.