The National Institutes of Health will allow funding for research to make some viruses more dangerous and easier to spread.
It lifted a moratorium that was in place for three years that essentially banned the act while it decided if it was worth the risk.
Experiments aim to see what actions will create pandemics. The new policy applies to dangerous viruses like SARS, MERS and influenza .
But some say these experiments "pose substantial risks to human life." And some worry the viruses may escape from the lab or be used as a weapon.
Others in the scientific community say they need to study the pandemics to better learn how they work.