This is just a strange tought. There are a lot of people in the world who had survived traumas and had never be helped for it, and someimes did not even realize that. I especially mean those situations, when traumatic ezperience is considered normal, as in many third-world countries.
For example, so many people are starving and can not afford any food, and see their family members dying of hunger and diseases. I think, that can be a traumatic event.
Or, there many countries where the milotary conflicts are going on right now. Obviously, a lot of people are traumatised there.
Or, the countless crime or accident victims.
Hell, not everywhere you can say the you are still disturbed by the memories of rape or car accident... In many places, people will say that you are just a whiner if can not forget this, since there are worse problems all around.

I can see my husband, who does have symptoms of PTSD, but does not realise that there is a problem, calling it "Normal memories about the war, everybody has that".
Everybody.
I guess, he met a lot of people with the same issue, who had never been diagnosed and helped.
I had seen people who survived the ecomomic hardship in their countries, and this is common for them to make large reserve of food thay can use if something happans (we call it "Russian syndrome", but other people have it as well).
And, just looking through the books, how many times had you read something like this:
-Still can not talk about ---(something).

I think, that if all this is true, the PTSD can be the world most common unrecognised disorder.