Say thank you and you do the same.
The reason this is, it's because essentially they are wishing happiness at this time of year and the religion factor is not most important. I know a lot of people who grew up to not be too religious but love Christmas because its time for family and friends to come close and share time, singing, sharing gifts, laughing and that warm cozy feeling you get. When someone wishes you a happy Christmas, they are saying, I want you to get a cozy feeling too with your fam and friends.
It's kind of like when someone says, "God Bless you" The religious factor is not there, its a polite way of having to be in this society and they are also wishing you well in getting better through your sickness.
Its all intentions here based off of love and care. So so thank you.