I don't know who originally suggested posters for the front door on this site but i want to say what a great idea and thank you!
As suggested I googled 'Someone in his house has a weak immune system' in images which took me to a variety of posters you can print for your door, including 'Someone in this house is undergoing cancer treatment'
Hope this is helpful
Thanks to the person who originally suggested this, whose post i can no longer find.