What can I do if my landlord neglected to address a gas heater issue until I became ill with CO poisoning a month later?

You could sue the landlord for negligence.  Negligence is the failure to exercise due care (that degree of care in this case that a reasonable landlord would have exercised under the same or similar circumstances to prevent foreseeable harm).  Prior to filing a lawsuit for negligence, it may be possible to settle the case with the landlord's insurance carrier.  When you complete your medical treatment and are released by the doctor, obtain your medical bills, medical reports and documentation of any wage loss.  Your personal injury claim filed with the landlord's insurance carrier should include these items.  Compensation for the medical bills is straight reimbursement.  Compensation for wage loss is straight reimbursement.  The medical reports will document the nature and extent of your injury and will be used to determine compensation for pain and suffering which is an amount in addition to the medical bills.  If you are dissatisfied with settlement offers from the insurance company, reject the settlement offers and file your lawsuit for negligence against the landlord.  If the case is settled with the landlord's insurance company, NO lawsuit is filed.  If the case is NOT settled with the landlord's insurance company, you will need to file your lawsuit for negligence against the landlord prior to the expiration of the applicable statute of limitations or you will lose your rights forever in the matter.