Either/or. Often times it's the primary disease, other times it's secondary. It is a disease of its own right, but can be the consequence of other diseases - whether psychological, neurological, or physical.

Tremor is maybe a more direct comparison than a cold. You can have essential tremor, in which tremors are idiopathic and the primary diagnosis. On the other hand, tremors can also be symptom of multiple sclerosis. It just depends on the individual.

Frequently, it's a chicken or the egg situation. Did the individual develop PTSD because they were already predisposed to/experiencing depression or did they become depressed after developing PTSD?

Sometimes depression is also just a natural consequence of being sick and/or disabled. Chronic pain, physical limitations, the inability to maintain a healthy social life, poverty as a consequence of disability - it can really take a toll on your mental health.