People cannot smell, taste, or feel carbon monoxide. Therefore, its presence may not be evident until symptoms develop. Symptoms of carbon monoxide poisoning include:
Rapid heart rate
Nausea and/or vomiting
Thought impairment or disorientation
Wheezing/bronchial constriction/pesistent cough
Chest pain in persons with coronary artery disease
Death can occur in the absence of some or all of these symptoms. In this case the victim simply "falls asleep" and never regains consciousness.
Source: Pennsylvania Department of Health