Dementia is a syndrome, not a disease. Dementia is a group of symptoms that affects mental cognitive tasks such as memory and reasoning. Dementia is an umbrella term that Alzheimer's disease can fall under. It can occur due to a variety of conditions, the most common of which is Alzheimer's disease
You might find this surprising, but the latest science shows that loneliness can be socially contagious. One factor might be that if you and your friends are feeling lonely, you’re obviously not connecting with each other that well.