i voted injury. I don't see it as an illness, because I see illness as something that is caused by chemicals or germs, or something related to the body going wrong - and to me PTSD is essentially caused by trauma which happens in life, not the body as such. However once PTSD develops, the body becomes affected by it so it's more like injury to me.
I don't see it as a syndrome because that from what I understand is typically a term that simply describes a set of symptoms that a cohort of people similarly have.
I kind of see it as a disorder... PTSD is disordered coping mechanisms, disordered memory, disordered arousal (hyperarousal) etc. etc. ... but it is also injury psychologically and that fits better with me. I would rather see myself as injured from trauma, and trying to recover, as opposed to 'disordered'.