By Richard Black
BBC science correspondent
Scientists in the UK say they've discovered how one of the most important types of malaria drug works.
Malaria kills millions each year
They believe their discovery will make it easier to monitor drug resistance, and to design new treatments for malaria, a disease which kills at least a million people each year.
The drugs are called artemisinins, and they're derived from a Chinese herb called quingao or sweet wormwood.
The herb has been used against malaria in China for centuries, perhaps millennia.
In recent years its extracts have become one of the most important types of malarial drug, as the parasite has evolved resistance to other kinds.
Dr Sanjeev Krishna from St George's Hospital in London led the new research.
He told the BBC: "This class, the artemisinins, does not seem to have any resistance against it in the parasites.
"So it's one of our last few hopes against treating drug-resistant parasites."
Dr Krishna's team has now discovered that the artemisinins work by disabling a vital part of the parasite's cells - a tiny natural molecular motor which pumps calcium from one part of the cell to another.
This discovery opens up the possibility of designing new drugs which work in a similar way - targeting this pump or others vital to the parasite's existence.
It should also make it easier for scientists to monitor whether the parasite is becoming resistant to the artemisinins - they can sample parasites and study the calcium pump - if it's changing, that would give an early warning that resistance is developing.
Dr Krishna added: "Up to now, people have thought that they worked like a dirty bomb.
"That is, they get into the malaria-infected red cell, and they go for the main factory which the parasite is using to digest haemoglobin, and they spatter around, and knock that factory out.
"But we've found in fact they behave like a smart bomb."
"It'll take some time to apply our findings, or even to test newer artemisinin derivatives which are being developed now. But you can be sure that's what we're going to be doing."
In a commentary in the journal Nature, in which the research is published, Robert Ridley of the World Health Organisation said the research had significance "beyond its undoubted scientific merit."