I would assume that, like other viruses and other bacteria, there is more than one microbe that can cause conjunctivitis, and there is more than one strain of the particular microbes that can cause conjunctivitis. (This is why not every staph A infection is ORSA; there are different strains of staph A.) Also, I'd be willing to say that these strains also mutate - so like continuously getting colds, you can continuously catch pink eye. Based on the roots of the word itself (and based on my physical assessment class on HEENT that I had three weeks ago) I'm discerning that the word means "inflammation/infection of the conjunctiva" - which by my own guess could be caused by a host of bugs.
And also remember this - like the other person said, just because you catch something once doesn't mean you won't get it again. Not all infections create strong enough memory cells (the creation of which varies from person to person) to prevent us getting the bug a second time; it just means you might not get it as strongly as you would have if you hadn't had it before. I was told that this is as much the idea of the flu vaccine as out-and-out prevention: many times you still get the flu post-vaccination, but it only feels like a cold because memory cells can fight the full strength of the virus off, and your symptoms are lessened considerably. Mutation also explains why people who have been vaccinated can still get whatever they were vaccinated against; also, some people never respond to vaccines in the first place, and only know it if they have titers drawn (isn't this not seroconverting?).
Even HIV has more than one strain - and you can be infected with both of them concurrently. That's why they tell HIV patients not to have unprotected sex with other HIV patients - you don't know what strain they have.
I'm just basing this on my own micro class and what I've had of pathophysiology so far this year. Anyone who knows infinitely more than me - and I know you're out there - please feel free to tell me I'm full of poo.
I will not be insulted at all.