Cancer may affect people at all ages, even fetuses, but risk for the more common varieties tends to increase with age.  According to the American Cancer Society, 7.6 million people died from cancer in the world during 2007.

Read more about Laughter and Cancer.

The most common types of dementia are Alzheimer's disease, vascular dementia and dementia with Lewy bodies.  Symptoms include loss of memory, confusion and problems with speech and understanding.

Read more about Laughter and Alzheimer's Disease.

Parkinson's disease is a degenerative disorder of the central nervous system that often impairs the sufferer's motor skills and speech, as well as other functions.

In recent years, researchers have shown that physical changes in the body can be accompanied by mental changes as well.

Medical illnesses such as stroke, a heart attack, cancer, Parkinson's disease, and hormonal disorders can cause depressive illness, making the sick person apathetic and unwilling to care for his or her physical needs, thus prolonging the recovery period.

Read more about Laughter and Parkinson's Disease.