I'm willing to talk about PTSD but only in general terms. It follows a very sudden and awful experience, as the name indicates. You have terrible auditory and/or visual flashbacks which just come out of nowhere. Any little thing, or sometimes nothing at all, can trigger these. They literally take you right back to the terrible event and you experience all the terror, pain, sense of helplessness and hopelessness that you experienced at the time. They leave you shaken and shaking and produce a huge adrenalin rush. Physically your pupils dilate, your heart beats too fast, your mouth goes dry. Emotionally you are a wreck. These flashbacks can last for years. In my case for three years, though I still have the occasional one.
I don't consider them a mental health issue as they are, to me, a perfectly normal reaction to an abnormal event. I didn't have and didn't seek any help. But then I am extremely resilient mentally having overcome many other crises in the past. Unlike PTSD, illnesses such as schitzophrenia, bipolar, and some forms of depression are mental illnesses as they usually result from a chemical inbalance in the brain.
However, having said that, mental illness can be like a physical illness inasmuch as if you have a suppressed immune system then a simple cold could turn into something much worse. Likewise, some people through no fault of their own, can find that PTSD can lead to depression and more. Strangely, it has been found that the relatives of victims of 9/11 who sought counselling did 'worse' than those who did not. This may be because those who already had coping strategies, and family or friends to support them, were therefore less inclined to seek counselling in the first place.
I hope this explains it a little nfk