Fernman - underlying health conditions at the moment seems to mean hypertension, COPD, type 2 diabetes and asthma for elevated risk. Presumably people are saying obesity because it correlates with diabetes.
Gunwharfman - CDC and NHS are treating this as droplet protocol for issue of PPE (mask, goggles). Not like measles where you are exhaling it in your breath, you need to cough, sneeze or wipe nose to move these larger droplets and pass it on to someone else. However once sneeze/cough those larger droplets can persist in still air for quite a while.
In practical sense anyone coughing around you is a risk factor, as well as surface transfer. Highly unlikely you'd have an issue with leaving car vents open.
Agree with everyone saying those without a car are going to have a hard time of it, both in getting supplies and actually getting out there.