guy shoots best friend

[Megan]

okay here's what I remember; just forgetting the name! 

It's a more recent movie. Something to do with some kind of disease that is spread by the air and contracted when in contact with an infected person. 

there are friends in a car driving in the desert trying to escape. They pass a man and his daughter stranded and try to avoid them. 

two guys, who are best friends. At one point one guy begs the other to shoot him because he has become infected. Now I'm not entirely sure but I think once infected, you're a zombie/vampire. 

any help, greatly appreciated.

 Thanks

[Debra]

There's a current film 'Contagion' about a deadly virus that's easily spread airborn, but there's no vampires or zombies in it...  people just get sick & die quickly. It has Matt Damon & Lawrence Fishburne in it... Good luck with your search.  

[thatboy]

The Crazies?

[Ethan Bourne]

Another possibility is Carriers (2009), where four friends are stranded in a desert when their car breaks down, and most everyone else is plagued by a viral pandemic.  They meet up with a man and his infected daughter; as they try to flee the epidemic, various disasters befall them ... :

In a world devastated by an outbreak, Brian, his girlfriend Bobby, his brother Danny and their friend, Kate, are heading to a beach where the brothers spent their childhood vacations expecting it to be a sanctuary. When their car breaks down on the road in the desert, they negotiate with a man called Frank, who is also stranded but without gas. He is trying to get his daughter Jodie to the hospital (she is infected with a disease of some kind), they all decide to travel together. During the journey, massive moral dilemmas arise, as events head into a downward spiral.

[Ethan Bourne]

Megan, here's Wiki's description of Carriers, which seems to fit very closely with your opening post.

Netflix has this on streaming here ...

As a lethal virus spreads across the globe, brothers Brian (Chris Pine) and Danny (Lou Taylor Pucci), along with Bobby (Piper Perabo) and Kate (Emily VanCamp), seek a supposed plague-free haven. But while circumventing those infected, the travelers begin to turn against one another. Real-life brothers Alex Pastor and David Pastor write and direct this apocalyptic chiller, which co-stars Christopher Meloni.

[Megan]

Thank you all for your quick responses.  And Ethan for both of yours. "Carriers" was the movie I had in mind. I remember the only reason I watched it was for Chris Pine. 

 Thanks!