A virus 'reprogrammes' cells in the lining of the lymph vessels and turns them cancerous, scientists have found.
Researchers investigated how the cancer started
The cancer Kaposi sarcoma develops in lymph vessels, which are the 'transport network' for the body's immune system.
It is common in people who are HIV positive or have had organ transplants, and therefore have weaker immune systems.
Cancer Research UK scientists, say in Nature Genetics, their finding may lead to new drugs to treat the cancer.
They say if drugs can be developed which arrest the development of cells lining lymph vessels, they could thereby stop the cancer developing.
It could also yield a test for doctors to predict which patients are likely to develop this cancer.
Kaposi sarcoma typically appears as coloured lesions or blotches on the skin. It commonly disappears if the immune system is restored, but can otherwise lead to potentially fatal intestinal and lung lesions
Due to its link with HIV and Aids, it is the most common cancer in many Sub-Saharan African countries.
The cancer was first identified in 1872 but, until now, it had not been known where it first developed.
The researchers analysed the genetic makeup of Kaposi sarcoma cells and found they were most similar genetically to cells of the inner lining of lymph vessels, known as lymphatic endothelial cells.
They also found that the virus that causes Kaposi sarcoma - the Kaposi sarcoma herpes virus (KSHV) - can turn the endothelial cells that line blood vessels into cancerous lymphatic endothelial cells.
It was also discovered that high levels of circulating growth factors, which encourage lymphatic endothelial cells to proliferate, were strongly associated with the development of Kaposi sarcoma.
Professor Chris Boshoff, who joint director of the Cancer Research UK Viral Oncology Group at UCL, who led the research, said: "Now that we know in which type of cell Kaposi sarcoma originates, we should be able to identify new ways to treat the condition.
"The findings could also yield new clues about other cancers that are triggered by viruses, such as cervical cancer."
Professor Boshoff added: "Finding new cancers early is vital if treatment is to be successful.
"By testing blood for growth factors that encourage the cells lining lymph vessels to grow, doctors might be able to predict which high-risk patients - such as those with HIV or who have had an organ transplant - will subsequently develop Kaposi sarcoma."
Professor Robert Souhami, director of clinical and external affairs at Cancer Research UK, said: "People with HIV or who are taking immunosuppressive drugs are particularly susceptible to cancer.
"The discovery that Kaposi sarcoma originates in the cellular lining of lymph vessels furthers our understanding of the disease and also has strong therapeutic implications.
"This is a good example of how research into the basic workings of cells and viruses can open new avenues of opportunity for treatment."