Think of dementia as a big umbrella symptom. Alzheimer's is just one cause/kind of dementia. Some forms of dementia, like Alzheimer's or Behavioral Variant Frontotemporal Dementia, are caused by faulty proteins in neurons (nerve cells). Other forms of dementia, particularly ones that affect younger people and children (yup, children can have dementia and it's horrible), can be caused by genetic errors in how cells work and metabolize certain substances. Brain injuries like strokes, particularly lots and lots of "mini-strokes", can cause dementia too. Some infectious diseases, like syphilis and HIV/AIDS, can also cause dementia. Then there's also prion diseases like Creutzfeldt-Jakob Disease, where a type of misfolded protein makes healthy/normal proteins in the brain get all twisted up, causing progressive and rapid brain damage.

So dementia itself is not a disease, but a symptom or result of a disease or injury affecting the brain.

It came out in 2001, but maybe Wet Hot American Summer?