Need Advice on Trip to Latin America

Hi. I'm interested in travelling in Latin America this fall for about a month or two.
I would like to visit Mexico and possibly work my way down to Colombia by bus. Am I nuts? Soy loco??? As of now I don't have a travel partner. Just wondering how safe this trip would be. And will I need vaccinations or shots? I would like to end up in Cartagena and fly back from there.