What should I do next if landlord neglected to place ventilation on gas water heater?

It is building code that the water heater must be ventilated. You can call and inform the city where you reside and report the problem; this is probably the best option. The city has a right to inspect the interior of rental properties likewise, you can consent to an inspection while you live there. If you know factually that the poor ventilation is causing the illness, then you can vacate (if the place is uninhabitable) and cease paying rent until the problem is fixed. Alternatively, you can fix the problem and back charge the landlord.

Your question is actually not as much about dealing with your landlord as it is about getting compensation for pain and suffering due to the actions of your landlord. Your contract will affect how you pursue your claim against your landlord. Take a copy of your lease to an attorney. They will need to identify any restrictions on lawsuits, such as a requirement to arbitrate instead of going to court. If there are no significant blocks in your lease to suing your landlord, your attorney can often take the case on a contingent basis, meaning that they will get paid only if you win. If you do file a lawsuit, your landlord may choose to settle for a specific amount of money. If they don't settle, you will spend several months or years working your way through the court system. Don't count on receiving the money from your landlord anytime soon, if ever. Make plans to deal with medical and income issues now without the landlord's assistance. Hopefully you can get compensation later. This response assumes that you have already approached your landlord with your problems. If you have not, now is the best time to do so. Your landlord may have insurance for this type of problem that will pay your costs. Or, in a few cases, your landlord may be willing to help simply because they feel it was their fault.