Aphasia, the illness that has sidelined actor Bruce Willis, is a brain disorder that leads to problems with speaking, reading and writing. The recent attention to the little-known condition has left many wondering about its possible causes.

A stroke, tumor, head injury or other damage to the language centers of the brain can cause aphasia. A brain infection or Alzheimer’s disease can trigger it.

Former U.S. Rep. Gabby Giffords, wounded in a 2011 shooting, has aphasia from that injury.

The National Aphasia Association estimates 2 million Americans are affected and nearly 180,000 are diagnosed with the disorder every year.

Willis' family announced Wednesday that the 67-year-old actor has been diagnosed with aphasia but did not provide any details on a possible cause.

For an actor, aphasia could pose a huge challenge depending on how severe it is, said Johns Hopkins University cognitive scientist Brenda Rapp, who works with people with the condition.

"You can imagine how frustrating it is if you can’t find words, if you can’t organize words into sentences, if you can’t get your mouth to produce the sounds you want it to produce," Rapp said. "You are still yourself ... but you may not sound like yourself."

For most, the cause is a stroke that has cut off blood to part of the brain. Without oxygen and nutrients, brain cells die, which leads to difficulty retrieving words.

Aphasia does not affect intelligence. Some people improve dramatically in a few months. Others may need to find other ways to communicate. Speech and language therapy can help.

Researchers are also looking into new types of speech therapy and noninvasive methods such as a procedure that uses magnetic pulses to stimulate brain cells.