This completely depends on what they could or could not be exposed to. If you are riding in a car with someone who has a contagious/communicable/infectious disease then there is a high possibility that you could become exposed to that disease. If you are riding in a car with someone who has just had radiation therapy from cancer treatment then yes there is also a possibility that you can become exposed to the radiation too. If you are riding in a car with someone who has an STI then no you will not be exposed.