Parasitic organisms which are transmitted via water can cause various diseases, including hepatitis A, diarrhoea, typhoid fever, cholera, amoebiasis, and some types of worm infestation. Infection can be carried through drinking contaminated water, if it contaminates food, swimming or wading in the water. World-wide the contamination of drinking water is a significant method of the transmission and spread of such disease. It is important to avoid swimming in polluted water as there is a risk of contracting disease if the water is swallowed. Rat's urine can contaminate water and cause a form of leptospirosis, and in tropical countries there is a risk of contracting schistosomiasis (bilharzia) caused by a fluke that can burrow through the skin of the swimmer.