Invisible illnesses are conditions patients have that are not obvious when looking at them. Often people with an invisible illness face a lot of prejudice; others accuse them of faking, lying or exaggerating their illness. People just don't fully understand what patients with invisible illnesses are going through. Just because you cannot see a person's illness doesn't mean they don't have one. Just because a person looks OK doesn't mean that they're feeling OK. Invisible illnesses often have no cure and patients need to take medication for the rest of their lives to help control and manage the symptoms. Here are some of the invisible illnesses that could be affecting your friends and colleagues that you wouldn't know about unless they told you (based on information from bustle.com): Cancer comes in many different forms and certainly for most cases you wouldn't know a person was being treated for cancer unless they told you. While chemotherapy can often make cancer patients lose their hair, other forms of cancer treatment leave no visible outward signs. Watch "Think Like a Scientist," a video about Alzheimer's disease.