Children play with the sap
The sticky sap of the African milkbush may be an important cause of Burkitt's lymphoma - the most common childhood cancer in much of Africa.
The milkbush (Euphorbia tirucalli) is a tropical plant that grows in many parts of Africa and in the Amazon rainforest of South America.
Africans use the white sap that gives the plant its name to make glue and herbal medicines, while children often play with it.
But scientists in the US have found that exposure to the sap may make children more susceptible to the effects of a cancer-causing virus.
It is common to see people growing the milkbush as fencing around their homes
Burkitt's lymphoma is a tumour of the immune system and is the most common childhood cancer in Central Africa, although in Western countries it is rare.
While scientists think infection with the Epstein-Barr virus is the major cause, they have struggled to identify other risk factors.
But several teams of researchers have noticed that rates of Burkitt's lymphoma are much higher in areas of Africa where the milkbush is common than where it is rare.
Scientists from the University of Michigan grew milkbush plants in the laboratory and studied how the sap affected the behaviour of Epstein-Barr virus.
The virus often lies low within human cells, without causing much harm.
But scientists found that even very low concentrations of sap seemed to switch it into a more active state - in which it was much more likely to trigger cancer.
Switching on genes
In particular, milkbush sap seemed to make the Epstein-Barr virus switch on three genes crucial for its ability to replicate and infect new cells.
It is this ability which is thought to enable the virus to trigger cancer.
Lead researcher Dr Rosemary Rochford, of the School of Public Health at the University of Michigan, said: "In countries like Kenya and Tanzania, it is common to see people growing the milkbush as fencing around their homes, making it into medicine, or children using the gooey sap as a toy.
"Yet we've found evidence that they may unknowingly be putting themselves in danger."
Sir Paul Nurse, director general of Cancer Research UK, said: "In Africa a depressingly high proportion of children die with almost no treatment, so it is particularly important to identify the factors that may be increasing their risk, in order to help prevent the disease.
"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."
The research is published in the British Journal of Cancer.