I've had MRSA in my forties. I was lucky, my primary care provider caught it before it had gotten bad enough to require hospitalization, strong, STRONG antibiotics took care of it. When I had it, it manifested in a fashion that I thought was allergies, mostly localized swelling. I ended up trying to treat with antihistamines but that didn't accomplish anything. As it got worse I went in and was immediately sampled for testing and started on both oral and topical antibiotics.
So yeah, it can happen naturally to someone that's otherwise healthy and doesn't have a history of this sort of infection. The trouble is, it's not especially common either, so when the second whistleblower dies during a short span of months once investigators are actually paying attention, it's not something that should be ignored. Unfortunately given the budget that Boeing and its rich execs and board members have, it would probably be trivial to find a way to pay an assailant to do something to whistleblowers that doesn't easily tie back to them so long as they're not stupid about how they transfer funds.