I don't know what it is with the work ethic culture in America but it seems like you end up with one of two kinds of people; those who understand and recognize that your mental and physical health is important and that if you are feeling unwell in any capacity you should take the time off of work to heal, and those who are stuck in the ways that your job status comes first and if you have a meeting or such planned that you are not allowed to take off for the day even if you end up going to the ER for your condition.
Especially knowing that everyone has different tolerance levels and different immune systems, different mentalities, different anything; you should be willing and able to acknowledge that everyone needs to take care of themselves differently for a similar ailment. You can make your own choices on if you feel up to coming to work but never judge somebody else for deciding they need to take off. It is not your place to make that call for them and it is not your place to judge them for doing so.
I am sure you would want the same respect and privacy as them so let them be and if you really want to be nice, ask what you can do to help them; be it making them a bowl of soup or picking up one of their cases for them so that they can have one less thing on their plate.