Children use the sap from the milkbush plant to make toys, but researchers believe exposure to the sticky liquid may make them more susceptible to the effects of a virus that causes Burkitt's lymphoma, a tumor of the immune system.

"It is a critical clue to what might be driving the high frequency of Burkitt's lymphoma in Africa. It also gives us an idea of how we can begin working on preventing the cancer in the children as well," Dr. Rosemary Rochford, of the School of Public Hygiene at the University of Michigan, said in an interview.

The milkbush is common in countries like Kenya and Tanzania, where it is grown as fencing and used in medicines. Children also use it to make toys.

But Rochford and her team found evidence that children may unknowingly be putting themselves in danger.

When they studied the impact of the sap on the virus in the laboratory, they discovered low concentrations switched on three genes that were important in various stages of the virus, allowing it to replicate, kill cells and infect new ones.

Their research is reported in the British Journal of Cancer.

Burkitt's lymphoma is a very aggressive disease that has been linked to the Epstein-Barr virus. Although children are most affected by the disease, adults can also develop it. It is usually treated with chemotherapy.

Scientists had previously noticed that illness rates are higher in areas of Africa where the milkbush is more common.

"Burkitt's lymphoma is found in western countries, as well as Africa, but you never see it in the jaw in western countries," Rochford explained.

She suspects that while playing with the sap children might be putting their hands to the face and absorbing it into the mouth and stomach.

Educating parents and children about the dangers of the milkbush could help to prevent the cancer, Rochford added.

"Further research is necessary to confirm the link between exposure to milkbush sap and Burkitt's lymphoma. But this study could be important if avoiding exposure to the plant reduced the number of children suffering from the disease," said Sir Paul Nurse of the British charity Cancer Research UK.