Can I break my lease or withhold rent because of an unsafe and unhealthy condition?

In your case, you need to send your landlord a letter, detailing the problem and asking them formally to fix this problem. (Mail this request return receipt requested and, of course, keep a copy of the request for your records).  If your landlord still refuses to make an adequate repair, you have several options.   

One action you can take is to make the repair yourself and take the amount you spend out of the next month's rent.  This is called "repair and deduct".  It is a good idea to get written estimates before making repairs, here having a plumber come in, and to write the landlord that you plan to take care of the leak if he doesn't.

Another way to get your landlord to exterminate is not to pay the rent until he fixes the problem. This is called "rent withholding".  Rent withholding is a serious step to take.  However, it is appropriate if there are bad health or safety problems in your home. 

Finally, it is your right as a tenant to have a livable, safe and sanitary apartment.  This is known as the "warranty of habitability" and is implied in every lease.  If conditions are such that a tenant's health and welfare are at risk, then this warranty has been breached and a tenant may terminate their lease early.  Contact the Town’s Building Inspector (also called a Code Enforcement Inspector), the Fire Marshall or the County’s Department of Health to look at the problem. 

Just make sure to have all letters to the landlord and any other evidence, for example, notice from the health department, estimates for plumbing repair, etc. at you disposal.  If your landlord tries to evict you or sue you for any remaining rent due under your lease, you will have to present all of this to the court.

However, before you attempt any of the above, you need to speak with an attorney experienced in handling these matters.  Legal Aid is a good place to start.  Even if you don't qualify for representation by them they should be able to give contact information to tenant's rights organizations in your area.  Best of luck.