Most human cases of bird flu in North America have been mild, a fact that’s underscored by a new study of the first 46 confirmed human H5N1 infections in the United States this year. But the case of an ill Canadian teen stands out because of its severity and because the source of exposure remains a mystery.

With the number of cases continuing to grow, leaders from the National Institutes of Health are calling for more action to tackle the bird flu outbreak.

The teenager, who was hospitalized with H5N1 infection in November, became critically ill and spent almost two weeks hooked up to machines that took over for her failing heart, lungs and kidneys, according to a report published Tuesday in the New England Journal of Medicine.