Hottest water you can get. And don't wash them with anything else.
I'm surprised some laundry soap company hasn't created instructions for using their detergent to disinfect cloth tissues (i.e. handkerchiefs).
It doesn't make sense to run a whole washing machine load to just wash a few handkerchiefs in scalding water, so there must be a method for filling a small bucket with hot water and detergent/disinfectant and letting them soak for a certain amount of time that's been tested to kill viruses and other germs.