Which of the following statement(s) regarding pneumonia is/are correct?
I. Only bacterial infection leads to pneumonia.
II. There are various forms of pneumonia that tend to cause an excessive build-up of mucus in the lungs.
III. Oxygen does not easily diffuse into the blood and carbon dioxide cannot diffuse out easily is one of the effects of pneumonia.
IV. Difficulty in breathing, fever, chills, cough and headache are a group of symptoms that indicate pneumonia.