But physical symptoms can be transmitted from person-to-person in small communities by seeing someone suffer or even from hearing rumors of illness.
"It's the reason why [mass hysteria] can spread almost like an infectious disease," Fricchione said. "If someone hears or sees a person with significant or severe symptoms, and if that is accompanied by a sense of risk transmitted by word of mouth or observation, the minds of those people will process that information in unison. ... A belief sets in that something terrible is going to happen to them."
In cases of community-wide illnesses, Fricchione said determining mass hysteria depends on the evidence. Public health officials investigate at the community level to determine a common cause while doctors who see patients individually might try to determine what disease process could result in the symptoms people are experiencing.
"But when people start to develop symptoms without the [causal agent], that makes it even more complex," he said.
Melissa Lenderman contributed to this report.