Another polio worker was shot and badly injured in Pakistan on Wednesday, December 19 the latest in a string of attacks that left 6 female health workers dead in 24 hours, Reuters reported.

The shooting in the northern city of Peshawar so soon after the other killings calls into question whether the U.N.-backed campaign to eradicate polio in Pakistan can continue.

It's unclear who is behind the killings.

The Taliban have repeatedly issued threats against the polio eradication campaign and health workers said they received calls telling them to stop working with the "infidels". But a Taliban spokesman said his group was not involved.

A global vaccination campaign has eradicated the disease from everywhere except Pakistan, Afghanistan and Nigeria.

Wednesday's victim was part of a team of four or five men administering polio vaccinations when gunmen opened fire on the group, said a doctor at Lady Reading Hospital where the man was being treated. He remains in a critical condition.

Pakistan had 20,000 polio cases in 1994 but vigorous vaccination efforts had brought the number down to 56 in 2012, the statement said.