Fact 1: Pneumonia may be caused by an infection or injury of the lungs.

Fact 2: Infections of the lungs can be caused by bacteria, viruses or fungi.

Given the two facts above, answer the question "Pneumonia may be caused by viruses or " with the following options: 
- bacteria
 - a hazard
 - cold weather
 - humidity
 - mildew
 - PDAs
 - injury
 - methane


 

       injury  