What are the 3 most common diseases in the world?

Who knows? Literally, the who (world health organization) probably knows. Conventional wisdom says periodontal (gum) disease is #1. Surely the common cold must rank up there. I suspect degenerative arthritis is high on the list if you include all age groups. As a cardiologist, i would put chagas disease on the cardiac list worldwide though very rare in the usa, thankfully .