i was playing a few games of Domination last night. I shot this guy on the back and i got the challenge Transaction Complete and after the game i got the STD title. Does anyone else have this? How long has this been happening to PS3 players as i thought it just affected Xbox and PC players? Does this mean that the unlimited ammo glitch or the public "private match" is affecting PS3 players now?

Lol no the STD title is a special title that is given to you when you stab a player using it.  I don't think you need to worry about glitches or anything like that, it's just a special title.

its the same as the couple of infected titles that spread by killing the people that have them as their current title. its meant to be in the game and its completly intended

As the poster above me already has stated this is not a glitch. This is one of them titles that you don't see in the Barracks. There are a few of those and if you want to see all challenges head over to this page (at the bottom of that page you can view the other/hidden challenges). If you'd like to see all the possible titles and emblems and how to get them click on this link.

I've been thinking about this, STD and Infected that is. Who started it?

Sorry bro, Beat to the punch by six days, Check this out, explains everything. 
 
http://www.giantbomb.com/modern-warfare-2/61-24713/infinity-wards-behemoth-beset-by-disease/35-378491/#5

As tech savy as we all are, it seems nobody knows what the actual definition of a glitch is.

llooool, same case for me, some guy killed me and it says i got infected, i got worried at first but then i realized its just a cool title lol
i started to use the infected title then kill others, they probably got infected too lol