Some life insurance companies will turn you down flat, while other life insurers may exclude the specific health condition.
For example, if you have COPD, and COPD is excluded form coverage, the life insurance policy would not pay out if you were to die from the disease. You would, however, be covered if your death was not related to your COPD condition.
There are a number of high risk life insurers that offer life cover to people with pre-existing conditions, but you should be prepared to pay a higher price because of the higher risk of death.
There are also guaranteed approval life insurance plans available for people ages 45 to 85 with no health examination required, no health questions asked, and you cannot be turned down due to any health problems.