It is hard to pinpoint the number of Americans with an invisible disability, but it's estimated there are millions. Their conditions may range from lupus to bipolar disorder or diabetes. The severity of each person's condition varies, and the fear of stigma means that people often prefer not to talk about their illnesses.
But in employment disability discrimination charges filed with the Equal Employment Opportunity Commission between 2005 and 2010, the most commonly cited conditions were invisible ones, according to analysis by researchers at Cornell University's Employment and Disability Institute.
"You know, it's that invisible nature of an illness that people don't understand," says Wayne Connell, the founder and head of the Invisible Disabilities Association. He started the group after his wife was diagnosed with Lyme disease and multiple sclerosis.
"We'd park in disabled parking and she didn't use a wheelchair or a cane, and so people would always give us dirty looks and scream at us," he recalls.
"When they see someone in a wheelchair, OK, they get that they're in a wheelchair. But what if they have chronic pain, what if they have PTSD — anything from cancer to peripheral neuropathy to autism?"