Am I responsible for my deceased husband’s medical bills, some of which have gone to collection?

As a general rule, one spouse is not obligated to pay the medical bills of the other spouse. Exceptions however exist to this rule. The first exception as to do with just where a spouse resides. If it is in a community property state, the surviving pouse would typically bear responsibility for such a debt. However, MI is not such a state. The second exception would be if a spouse agreed to be legally bound for re-payments on the other spouse's the debt.  For example, if upon a spouse’s admission to the hospital, the other spouse signed papers that would obligate them for payment of any hospital bills. The last exception falls under something called the "Doctrine of Necessities" (as you seem to already be aware of). Under this doctrine, one spouse is liable for the "necessary" expenses incurred by the other spouse during marriage. This holds true for any debt but particularly for medical bills which are deemed to almost always be “necessary”. Fortunately for you, MI no longer follows this doctrine.

Note:  In the case of a deceased spouse, even in a situation that did not fall into one of the above exceptions, the deceased’s estate would still be liable for repayment. Therefore, indirectly a surviving spouse could be affected financially.