A nine-month investigation by Al Jazeera has revealed that Yasser Arafat was in good health until he suddenly fell ill on October 12, 2004.

Further tests revealed that Arafat's final personal belongings – his clothes, his toothbrush, even his iconic kaffiyeh – contained abnormal levels of polonium, a rare, highly radioactive element.

Those personal effects, which were analysed at the Institut de Radiophysique in Lausanne, Switzerland, were variously stained with Arafat’s blood, sweat, saliva and urine.

The tests carried out on those samples suggested that there was a high level of polonium inside his body when he died.

So what does Polonium do to a person?

It is actually harmless when it's outside the body, but inside, it becomes one of the deadliest substances known.

An amount equivalent to the size of a particle of dust is lethal.

After being taken into the body, Polonium quickly gets into the blood stream. Then it bombards people's cells with millions of radioactive alpha particles.

It damages the organs - first the liver and the kidneys, causing jaundice. It damages the intestines, cuasing toxic shock syndrome. And finally, it attacks the heart.

Al Jazeera talks to David Barclay, a forensic scientist based in Edinburgh, Scotland.