Lake Malawi is breathtaking with clear blue water but, like many lakes and rivers in Africa and the Middle East, it contains a hidden danger.
Schistosomiasis or bilharziasis, a worm infection, can be caught by wading or swimming in fresh water in endemic areas.
Minute aquatic snails shed tiny parasiticworms. These are attracted to humans in the water and painlessly penetrate the skin.
Inside the body, they reach the bloodstream and pass through the heart and lungs to develop in the blood vessels of the liver and intestine or bladder and genitalia.
The adult bilharzia worms live for three or more years. They lay hundreds of eggs each day.
Some of the eggs pass out in the infected person's faeces or urine (and can infect more snails) but many are trapped in the body and cause problems.
The body's reaction to the eggs causes scars that gradually result in liver or bladder problems.
Many people with bilharziasis are unaware of their infection. Those with many worms can develop chronic liver problems or bladder symptoms. To diagnose the disease, urine and faeces are examined for eggs.
A blood test is often positive even if eggs can not be found. A tablet, praziquantal, is an effective treatment.
Bilharziasis is found in 74 countries. It is widespread in virtually all countries in Africa and the Middle East. Other areas include some parts of China, Cambodia, the Philippines, Brazil and Venezuela.
The only completely effective method of prevention is to avoid contact with fresh water.
Malawi has been the site of infection for many travellers, so don't believe signs or guide books claiming that Lake Malawi is schistosomiasis free. Think twice about rafting trips too.
