Carbon monoxide is a colorless, odorless and tasteless gas that can be deadly if not detected.  It is caused by an incomplete combustion process in a fireplace due to incorrect operation, a defect or a blockage, e. g. animal nests or other blockage in the chimney.
When inhaling, carbon monoxide prevents oxygen intake and oxygen deficiency arises in the body.  Symptoms include weakness, dizziness, headache, nausia, confusion and loss of consciousness.  If high levels of carbon monoxide are inhaled, the time frame from the appearance of the first symptoms until the loss of consciousness and death can be very short.