Pimples on the chest and back? What is this? Dangeruos?

Pimples. Depends. Recent or long standing? Usually it represents a form of folliculitis. Have your doctor culture the pustules to see what bacteria is causing them. Your doctor can then prescribe appropriate therapy.
Folliculitis. It sounds like you have folliculitis, a minor superficial hair follicle infection. It can be caused by bacteria, yeast, or fungal overgrowth and infection. Usually, it is not serious and easily treated. Sometimes it comes after using a contaminated hot tub (hot tub folliculitis) and is caused by pseudomonas bacteria. Try benzoyl peroxide (bpo) body wash for 1-2 weeks, seek help if not better.