The federal government announced on Tuesday that it is lifting a three-year moratorium on funding controversial research that involves genetically altering viruses in ways that could make them more contagious, more deadly, or both — and that critics say risks triggering a catastrophic pandemic.

Called gain-of-function experiments, the studies aim to understand genetic changes that can make viruses such as bird flu, SARS (severe acute respiratory syndrome), and MERS (Middle East respiratory syndrome) more transmissible from person to person. But if they escaped from the lab, perhaps through human error, the modified viruses could in theory spread quickly or be extremely virulent, increasing the toll of an outbreak.