If your state or county doesn't have a temporary ban on gatherings of 10 or more, and isn't implementing a "work from home'" and "stay at home" mandate for all non-essential workers, then the best you can do to protect yourself is to practice social distancing (6 feet apart) and wash your hands after touching any community surfaces she and others have touched.

There comes a point where you have to protect yourselves, if others are not respecting your right to a healthy work place. (Hold your breath when passing by her, if she ends up coughing/sneezing?)

I find myself holding my breath when people are getting too close to me -- First it was because I was worried I might be contagious after being sick -- now it's to protect myself from ever being that sick again.

Maybe set some sanitizer out in your workspace to show that you are being dilligent yourself in not spreading anything and encourage co-workers to do the same - to raise the bar a bit as far as your office's collective efforts in keeping everyone healthy?

It is not spread by merely breathing. It has to be physically carried from person to person. I have given a long explanation below.

Best to mind yourown business on this IMO, it is no different than going to a store. If she mentions anybody was unwell there, I would immediately DC lately go to HR or the boss and tell them, though!

Retire

...and thinks if she goes she goes.

She's basically forcing her attitude about this onto everyone else, which selfishly ignores the reality of others opinions about life and death. I'd talk to her superior about it with a "she stays home or I do, and btw I'm telling everyone here exactly why if I'm forced to do this" themed conversation.

If you were in Washington your office would probably be closed, that gathering "illegal".
Some of us on the coast where we're taking aggressive means to dispel this virus are wondering what "normal" is going to look like. We're told 6-8 weeks more of this shut-down and we'll be back to normal.
Normal being the cruise ships start-up again and we have 1000's of folks from states now doing nothing, rolling through Seatac Airport.

Here we are not allowed to have gatherings over 10 and most won't even do that. The 2 cases so-far were on the big island and both were in the same family. There has now been 1 confirmed case on Lopez (out of 2400 residents) and even though the name can't be given it has been said all those that were in contact with this person have been found and examined. All it takes is one infected person. Multiply, the number of people around you to multiply the odds of infection. If you're co-worker went I would stay away from the co-worker at work.
At our 'stage-of-life' we need to be overly cautious.

Like you I'm not concerned about the going but the manner this virus takes people out in. Gasping for air and choking are not on my list!