Yeah, if you smell gas it's best to get out of the house and alert your local gas utility company to come out and check for gas leaks.
Just about 4 or 5 years ago a building right here in Winnipeg was reduced to match sticks because of a gas leak.
Carbon monoxide is a colorless odorless gas that results (as previously mentioned) from incomplete combustion of natural gas. What happens is that if your chimney is blocked and the flue gas can't get out, then as the level of CO2 inside your house increases and the level of O2 decreases, then eventually the burning of gas results in the production of CO instead of CO2.
My understanding is that CO kills you by attaching to the hemoglobin in your blood. Basically, it takes the place that the O2 otherwise would, and sticks to that hemoglobin so that it can't expell the CO that's stuck to it. That drastically diminishes your blood's ability to carry oxygen to your brain and heart muscle cuz it's the hemoglobin that normally does that. As a result, your body suffocates for lack of oxygen.
CO is a colorless and odorless gas, you can't smell it or see it. You can buy CO detectors for your house, just like you can buy smoke and fire alarms, so that if you have a combustion problem in the middle of the night when you're asleep, the detector will alert you to the danger. Otherwise, you might never wake up.