Trauma can take many forms. A traumatic event could be something that happened to you, or something you saw happen to someone else. Seeing the effects of a horrible or violent event can also be traumatic--for example, being a first responder after a terrorist attack.
You are not alone. Going through a traumatic event is not rare. At least half of Americans have had a traumatic event in their lives. Of people who have had trauma, about 1 in 10 men and 2 in 10 women will develop PTSD.
There are some things that make it more likely that you will develop PTSD--for example, having very intense or long-lasting trauma, getting hurt, or having a strong reaction to the event (like shaking, throwing up, or feeling distant from your surroundings).
It's also more common to develop PTSD after certain types of trauma, like combat and sexual assault. But there is no way to know for sure who will develop PTSD.