About 80 percent of pneumonia cases are caused by viruses and 20 percent by bacteria. Viral pneumonia is usually milder than bacterial pneumonia.
Pneumonia is usually a complication of a cold.
Although, colds can be passed from person to person, bacterial pneumonia is not considered contagious. Only bacterial pneumonia is helped by antibiotics. Because it's difficult to distinguish bacterial from viral pneumonia in all cases, sometimes antibiotics are prescribed for viral pneumonia. Before antibiotics were available, bacterial pneumonia was dangerous. With antibiotics it improves within 24 to 48 hours.