Defeating smallpox has been labelled as one of science's greatest success stories.

The disease once killed 30% of those infected, but after a global vaccination campaign it was declared eradicated in 1980.

However the variola virus, which causes the infection, is not gone. It exists in two laboratories, one in the US and the other in Russia. The question is about to be asked, once again: should they kill their stocks?

The World Health Organization (WHO) will come to a decision at the 64th World Health Assembly this week.

It is not the first time the issue has arisen, it was first discussed at the Assembly in 1986 and has been the source of debate ever since.

Are stocks still needed?

Destroying the remaining stocks is seen in parts as the final chapter in eradicating the disease, otherwise there is always the risk of accidental release....