PTSD does occur in bystanders. I got it from watching my brothers and sisters get beat. I didn't get beat.

PTSD occurs in soldiers who watch horrible events without being IN the event.

So, I suppose people could get PTSD from observation alone.

As a child, and as an adult, long before I was diagnosed with PTSD I could never ever watch horror movies or scary movies because I would have severe nightmares for days to weeks on end. That was from watching a movie on TV.

So my dad refused to let me see the movie Psycho because, "you'll never take a shower again." At which point I stopped taking showers with the curtain drawn.

Anthony Hopkins who played what's his name in Silence of the Lambs went to Jamaica to have a friend for dinner. NEVER going to Jamaica.

Some people are likely more susceptible to PTSD and do not necessarily need strong events to trigger the symptoms.

One really does not need a "professional" diagnosis if you have a trigger that gives you nightmares all night long for days to weeks on end. That would be diagnosis enough for me. BTW, I was professionally diagnosed after 3 weeks of no sleep from terrifying nightmares.