Infection spread by Priors of the Ori. Afflicted individuals experience symptoms characterized by high fever, chest pain and respiratory distress. The envoys of the Ori use the plague to convert an entire planet to Origin through obedience (ridding the plague after everyone has capitulated), or to neutralize a society that refuses to relinquish their will.
Priors are capable of manipulating their own D.N.A. and passing the affected elements through touch. This is useful if the recipient is traveling to another world through the Stargate, or another distant location where the Prior does not wish those encountered en route to be infected.
The plague appears to be highly adaptable. It can apparently also be spread in an airborne fashion -- not necessarily through physical contact. What determines this is the task that is required of it. If it is meant to lay dormant for a period of time in a carrier and then generate as much damage as possible, touch seems to be the method of choice by the Priors.
The Powers That Be - After the population of P8X-412 is not swayed to Origin, a Prior infects the people with the plague.
The Fourth Horseman, Part 1 - Knowing Lieutenant Fischer is from Earth, a Prior appoints the officer to be a carrier for a subtle version of the plague. In days, thousands of humans are infected.
The Fourth Horseman, Part 2 - The Prior plague ceases its grip on Earth when a vaccine is devised, thanks to the efforts of Orlin and the Jaffa Prior, Gerak.
Ripple Effect - Team members from a parallel universe arrive at Stargate Command and report that their Earth is being ravaged by the Prior plague.