Actually I think that's a very intriguing question. The only thing I can think is that Jenner felt that Rick might accept the inevitable explosion if he knew the "secret". If that's true, though, why wouldn't he have told everyone? I think I just talked myself out of my own explanation

 

Good question...maybe he just felt like he should know. Maybe he'd wanna stay if he told him...or he just wanted him to know

 

He wanted Rick to know and maybe he figured it would be easier for Rick to know it in secret since they were all leaving anyway. He had Daryl trying to kill him, and Shane discharging firearms in the building. I might have kept it on the down low also.

 

Eh, who knows...it was season one...there were many mistakes in season 1

 

I saw it as his last ditch effort to get Rick to join his little suicide pact. A "hey, you know it's hopeless out there, you're all infected, why not join me and go out with a bang?"

 

I think Jenner was telling Rick that they're doomed anyway. You could try to escape the CDC, but there's nothing you can do to stop whatever it is that's ended the world. You're screwed.

 

I think that's when Jenner told them they were all infected and would turn into zombies / walkers if they died and their brains were not pierced.
Maybe Jenner just wanted some more company when he decided to commit suicide, the more the merrier.

 

We know what he said, we're just speculating on why he said it lol.

 

I think Jenner just didn't want to die alone, perhaps he was afraid or a coward, that's what I meant by "more company" and "the more the merrier".

 

I got the impression he was a bit of a coward too.

 

Well Jenner lost everything so he wasn't a happy camper, Jenner lost his brilliant wife. He may have wanted everyone else to share his misery along with him.
All the other scientists abandoned their posts at the CDC and left to be with their families so he probably wanted someone to die with.

Jenner believed their was no hope for a cure and living in the Z.A. was worse than dying.

 

I really don't think that the writers had a clue exactly what it was that Jenner was telling him when the episode was written. I believe that they just saw an open door for later on in the series to further explain the virus and knew there wasn't going to be another character like Jenner whose word would carry as much weight.

What leads me to this conclusion is the scene on the highway with so many people in their cars that never turned. I'm sure there are people that may show up and say that they did turn and I just couldn't see a head wound, but I couldn't find a way to explain it that made sense to me other than that it was a mistake on the writers part and the everyone is infected part wasn't written into the series until later on.