Thankfully, it hasn't been necessary so far for anyone to perform surgery in space. But in order to better figure out how surgery would be conducted in space, a group of scientists are carrying out simulated surgeries on a special Canadian research jet that simulates weightless conditions.
While surgery on Earth is complicated enough, space surgeries would be complicated to a whole different degree due to the absence of gravity.
So whilst no astronaut has gotten a severe illness so far while in space, the law of averages would suggest that it is just a matter of time. That, and the impending rise of commercial space travel, means it's important to figure out how to carry out surgery in space, which is why operations like this one are so important.