The assignment from Nichole Ward’s microbiology professor was simple: Choose a location, open a petri dish for three minutes and observe what grew over the next two days.

No one’s sample came back clean — a foregone conclusion given that a petri dish opened in any nonsterile room will collect microbes from the air. But when Ward returned to class with a dish that she had put in an enclosed Dyson hand dryer in a women’s restroom, the colonies of fungi and bacteria that had grown in it outstripped anything her classmates had found in their chosen locations.

“Mine just had so much more mass in the fungal growth,” she said in a phone interview Thursday. “Their little colonies were just a speck here and a speck there. It just stood out by far.”

Without further testing, which the class did not do, it is impossible to say whether the organisms in Ward’s petri dish were harmful to humans. But when she posted a photo on Facebook at the urging of her classmates, more than 500,000 people shared it in a matter of days.

Many horrified commenters vowed never to use a hand dryer again. Others ridiculed what they called unscientific fearmongering. Ward, who is taking the microbiology class as a prerequisite for a nursing program, said she had even received death threats.

Dyson, the manufacturer of the hand dryer she used, said in a statement to ABC that it was “very surprised to see these results, and unclear on the methodology employed.”

“All Dyson Airblade hand dryers have HEPA filters that capture particles as small as bacteria from the washroom air before it leaves the machine,” the company said. “Dyson Airblade hand dryers are proven hygienic by university research and are trusted by hospitals, food manufacturers and businesses worldwide.”

Whether hand dryers do, in fact, spread pathogens is a matter of dispute among scientists.

“The hot air will kill the bacteria on the hands, but some studies have found they will also deposit bacteria in the restroom on your hands — i.e., from the air,” said Charles P. Gerba, a microbiologist at the University of Arizona.

A study published in 2016 in The Journal of Applied Microbiology found that jet air dryers — high-powered machines like the one Ward used — contaminated the surrounding area with 1,300 times as many viral particles as a paper towel would. Standard hand dryers — those that simply blow warm air — spread far fewer particles, but still 60 times as many as a paper towel. (Dyson said when that study was released that it had been conducted under unrealistic conditions: Participants’ hands were thoroughly coated with a virus, which would not be the case for a typical pair of just-washed hands.)

A similar study in 2014, using a bacterium instead of a virus, found that jet air dryers spread 4.5 times as much bacteria as warm air dryers, and 27 times as much as paper towels — but the study was funded by a trade association for paper manufacturers. Another study published in 2010 found that jet air dryers resulted in fewer bacteria on the hands than warm air dryers — but it was funded by Dyson.

One of the few independently funded studies on the subject, published by the Mayo Clinic in 2000, found no statistically significant hygienic difference between dryers and paper towels.

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