If aliens ever came to our planet, they'd probably assume the place was run by the parasites.
Our whole ecosystem is run to suit them. By implanting itself in a specific area of the brain, it is perfectly possible for a parasite to control our behaviour, to ensure we live, eat, shit and die for the benefit the parasite and not ourselves.
Did the parasites invade Oldton, causing everyone in the town to act irrationally? Did they perhaps throw themselves of a cliff en masse like lemmings? Or, like my father, did they elect to end it all alone in the middle of the night?
Perhaps the switch that went off in his head was a parasitic infection, and not depression at all.