What are laws on landlord’s responsibilities when it comes to bedbugs in more than one of their units?

There is something called the "implied warranty of habitability," which is a legal obligation placed on all landlords that the premises they provide be "fit for their intended purpose"--for example, fit to live in. Significant pest infestations can violate this warranty.

When the warranty is breached or violated, the landlord has to take reasonable steps to correct the situation. Note that these steps do not have to work perfectly or the first (or even second) time--as long as the landlord is reasonable trying (e.g. bringing in licensed professionals) to deal with the situation, he or she is complying with this obligation.

If the landlord stops taking steps to deal with the problem, though, without satisfactorially resolving it, that would violate the implied warranty of habitability. In that case, you could potentially do one or more of the following:

1) Break your lease without penalty, if the condition is bad enough;

2) Bring in your own exterminator, then deduct the cost from rent;

3) Sue for monetary compensation for the time you've been living with bedbugs; or

4) Seek a court order directing the landlord to fix the problem.

If it comes to this, seek an attorney's help. (If you can't afford one, trying contacting Legal Services.) If you try to hold the landlord legally accountable for breaching the implied warranty of habitability when the condition isn't bad enough, the landlord is taking all reasonable steps, or you did not provide the landlord proper notice of the problem, you can find yourself liable to the landlord; therefore, it is better to let an attorney help you do this, if at all possible.