RI is just what it sounds like an infection of the lungs. It is basically pneumonia in reptiles. It is caused by bacteria virus or fungi it can be transferred by contact or airborne particles. Typically healthy well kept animals don't get sick with it. Often it requires some external aspect to generate conditions for it to take hold. Low ventilation and high humidity and warm temps will increase bacteria growth and increase the chances of RI. Low temps will depress the immune system of reptiles and increase the susceptibility. Temp shock, stress, injury can all contribute to an animal contracting some sort of RI. Sometimes having it once will leave an animal susceptible to it. There can also be some underlying lung related issue that acerbates it.
The guy before me had me surprised that he was even alive. There were pebbles from his driveway as bedding with waterbed heater inside the cage without the waterbed and the cage was littered with old skin and feces.