不是很容易, 和年齡身體也有關係

I was faced with similar problem two years ago. Two problems: 1) most places do not want to cover visitors that have not stayed for more than 1 year already, which is impossible if your parents come with B visas. I guess the reason is that they don't want to have people coming to US for the purpose of medical. 2) Rarely, you can find places that are willing to provide health issurance, but that also has lots of preconditions on current health status. Also, it tends to be very expensive (>$600/month, depending on age, health). Unless you suspect major condition to likely happen, it is not worth it.