Disease occurs in most tropical regions (affects 200,000,000 people world-wide), caused by a parasite called bilharzia (schistosoma). Acquired by bathing or wading in infested lakes, rivers & irrigation systems. Symptoms vary, but usually start with itchy rash ('swimmer's itch'), then {bleeding}, {ulceration} & {fibrosis} in {bladder}, {liver} or {intestinal walls}.
There is no vaccine against the disease, therefore travellers visiting tropical countries should avoid swimming in risk areas (i.e. lakes, rivers & irrigation systems).