A large-scale screen of herbal remedies in malaria-infected animals suggested that an extract from the wormwood species Artemisia annual might work. The results were inconsistent, so Prof Tu read ancient Chinese literature and found clues in Ge Hong's Handbook of Prescriptions for Emergencies, written in about 300AD. This guided her successful effort to extract the active component from the plant.
She was the first to show that this compound, later called artemisinin, killed the malaria parasite in infected animals and in humans without serious side effects.
Today artemisinin is prescribed in all malaria-affected parts of the world. When used in combination therapy, it is estimated to reduce mortality by more than 20 per cent overall and more than 30 per cent in children. In Africa alone, this means that more than 100,000 lives are saved annually, according to the Nobel citation.
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Professor Omura, who has carried out microbiology research at Kitasato University since 1965, looked for new antibiotics by cultivating Streptomyces bacteria, which live in soil. From thousands of different cultures, he selected about 50 of the most promising for further analysis of their activity against harmful microorganisms. He then sent these to Professor Campbell, who worked from 1957 to 1990 at the Merck Institute for Therapeutic Research, part of the US drug company.
Prof Campbell found that a component from one of the cultures was remarkably efficient against parasites in domestic and farm animals. The bioactive agent, avermectin, was purified and later chemically modified to a more effective compound, ivermectin.
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In 1987, Merck offered to make ivermectin available free of charge in all parts of the world plagued by parasitic worms. Since then treatment has been so successful that river blindness and lymphatic filariasis are on the verge of eradication, according to the Nobel statement, "which would be a major feat in the medical history of humankind".
In an interview with Japanese broadcaster NHK, Prof Omura said: "I have learned so much from microorganisms and I have depended on them, so I would much rather give the prize to microorganisms.
"I hope the area gets more attention because of the prize so that it can further contribute to human beings."
The Nobel Prize for Physics will be announced on Tuesday.