Dementia is an illness of the brain. Memory gets weaker, and little by little the person finds it harder to think, plan, talk, and look after themselves in the old way. It is not one single disease; several illnesses can cause it. Alzheimer’s is the one people hear about most. It usually comes on slowly, more often in older age, though not only then. Doctors use dementia as a careful medical word. Families often say “losing their memory” in kind everyday talk. Though actually, casual English also uses demented to mean “wild” or “crazy,” as a joke or an insult. That loose use is not this illness, and it can hurt. The medical word is about a real change in the brain, not about a person being silly or difficult on purpose.