If you are on a bus and someone smokes before they get onto the bus can the way they smell cause health problems?

Yes - this is referred to as "third-hand smoke". It is basically the toxic chemicals in smoke that stick around even after the smoker has put out the cigarette, cigar, or pipe. It gets trapped in hair, skin, fabric, carpet, furniture, and toys, and it builds up over time. Each time someone smokes, more smoke gets trapped in the fabric, furniture, walls, and other things around them. The chemicals from the trapped smoke pollute the air and get into people's lungs and bodies.

You can read more about it in this NYTimes article: http://www.nytimes.com/2009/01/03/health/research/03smoke.html.