Parasitic worms have been used as a medical treatment for various diseases, particularly those involving an overactive immune response. As humans have evolved with parasitic worms, proponents argue they are needed for a healthy immune system. Scientists are looking for a connection between the prevention and control of parasitic worms and the increase in allergies such as hay-fever in developed countries. Removal of parasitic worms from areas is correlated with an increase in autoimmune disorders in those areas. Parasitic worms may be able to damp down the immune system of their host, making it easier for them to live in the intestine without coming under attack. This may be one mechanism for their proposed medicinal effect.