You are presented with the quiz "After an infection or injury to the lung, what can happen?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "pneumonia may be caused by an infection or injury of the lungs" and "opportunistic infections, such as Pneumocystis carinii pneumonia, begin to occur". 

So, what's the best answer to the question?


 

      opportunistic infections     