For unpaid bills, can a hospital take your house while you are living or after you die if you have willed the house to your son?

If you have a medical lien on the house based on a collections judgment, a hospital can call in that lien theoretically and force a sale but more likely than not, it won't.  However, clear title cannot be passed on to your son and therefore, depending on how the property is willed to your son, your son may be able to take it free and clear of any liens (subject to as opposed to assumptions) if the estate documents are prepared properly.  Otherwise, the lien may be something the hospital (or collection agency) would have to follow by filing a claim through the estate.

Of course, you need to look at the time of the collections/lien and if you had already created a will and included this particular property in said will.  Contact your estate planning attorney to go over the timing of the transactions. 

A creditor, including a hospital, could, if they sue you and win, put a lien on the house. If their is a lien, several things happen: (1) potentially, a sale of the property could be forced, though that is rare; (2) if the house is sold subject to the lien, the creditor gets their share of the proceeds--the lien must be satisfied to pass clear title; (3) if the house passes by will or intestate succession, it would likely pass subject to the lien, which means that the person inheriting it would now have a property with a lien on it, to which he could not pass clear title, thus encumbering the property and making it more difficult to sell.