If there is a provable infestation of insects/rodents/etc, it is within reason to break the lease and seek restitution of rent paid. In order to do so, you must first contact the local board of health and have an inspection, then contact the housing court. I strongly recommend you speak with an attorney who handles landlord tenant matters.
I am unsure whether the laws of this state require a Landlord to disclose this type of thing, but surely where you specifically inquired about insect infestation before signing the lease, yet within days after moving in, you had such a severe and medically documented illness as a result, the Landlord would be a fool NOT to refund any monies paid, and allow you to terminate the lease.
I suggest you obtain copies of your trip to the Emergency Room, and send that, together with a letter requesting the relief you seek, to the Landlord, both by certified mail and by regular mail (USPS, first class, postage pre-paid).
You should also contact one of the legal clinics operated by the many law schools in the Boston area for further advice, if money is an issue.
I fail to see how Landlord can blame you, when you asked the specific question, and you do not state how he “justifies” this, however, also be sure NOT to complete the “tenant condition form” without including this specific complaint/issue, since this may impact your ability to recover monies paid and your ability to terminate the lease agreement.
Certainly you should not pay this unscrupulous landlord any further monies, and while, in theory, you should be paying those into an escrow account, if your medical documentation is as very clear as you state, I see no reason why you should not be able to use those monies to secure acceptable housing, while this is being resolved.
Best of luck.
No attorney-client relatonship is created in responding to this question, and advice provided is based solely on very limited facts presented, and therefore may not be correct. You are advised that it is always best to contact a competent and experienced with the practice of law in the county in which you reside, particularly as it relates to family law, child support, custody and visitation (a/k/a "parenting time") issues, including 209A abuse-prevention restraining orders (a/k/a "ROs" in legal-speak), regarding un-emancipated children, under the age of 22.