This was terrifying. How does an infection like this happen though? What can you do to prevent it?
Stay away from places where a lot of different people touch a lot of different things - and if you have to touch those things, wipe them down first with antiseptic wipes or similar.
In addition to this - take care not to let (or get, if possible - not always easy, depending on what you do for a living, your hobbies, and just life itself) wounds happen on your skin or mouth. If you do get a wound (cut, scratch, bite, etc) - try to take care of it properly immediately; clean it, apply dressing, etc. In the case of mouth wounds (ie - you bite your tongue or lip hard enough to draw blood), about all you can do is a salt rinse and keep your teeth brushed.
Stay away from hospitals and care homes and the like - seriously, one of the best places to get an infection is in a hospital or doctor's office. It seems counterintuitive, but when you have a place where sick people congregate, not everything is going to be "squeaky clean" even when they are regularly cleaned.
You might take the extreme step of wearing some kind of mask to reduce the inhalation of bacteria and/or viruses. It won't completely stop it, but it will reduce it a little.
Don't hang around sick people? If a coworker shows up to work sick, you might want to go home yourself (explain to your boss why, first - and let them make a decision as to who should go home - likely, they'll send the coworker home first). Unfortunately, so many people have kids, and are "carriers" of stuff that don't affect them much because they do have kids, but if you don't have kids and/or don't hang around kids much - you can pick up things that way easily.
Always wash your hands thoroughly after using the bathroom. Elbow out the door if possible (or use a paper towel to open the door).
Avoid stress (easier said than done for some people) - ultimately, though, stress does weaken the immune system; this is known. Better is to understand when you are stressed, and what forms stress can take, so you can know when you are under it. Some kinds of stress aren't always easily apparent. Note that this might also apply to "good stress"; I'm not sure. If you are tired and "stressed" from an extreme workout, I am not sure if that helps or hinders your immune system during your recovery...
Ultimately, though, you can't control everything, and you have to live your life. There's probably no way to trace back to where this guy picked up his infection. If I had to make a guess, maybe the gym? It could have easily have been during a run or a walk outside as well. Or just some door he pushed open to go into a room or building. The bacteria/viruses that cause these kinds of infections and issues are all around us, and our bodies do a fairly good job of keeping them at bay. But sometimes, bad luck happens.