15 years ago I remember zapping to a SciFi series on the TV that I don't know the name. It was about some humans ending up being possessed by a nanovirus or an alien virus. The only scene I remember is 2 guys standing up face to face : an old guy willing to infect a younger man (I suppose the main character) with a green fluorescent drop of something on the tip of one of his fingers.

I've tried to google these details though to no avail.
Anyone knows where it is from? X-files? The outter limit? which episode?
I would be very grateful for that