Flu (influenza) is a respiratory infection caused by several viruses (including “bird flu” and “swine flu”). Flu symptoms can be very serious, especially for the elderly, newborn babies and those with chronic diseases. Pneumonia is an inflammation of the lung that causes coughing, difficulty breathing, and fever. Other lower respiratory infections include tuberculosis, bronchitis, SARS, HPS, croup, diphtheria, pertussis (whooping cough) and more.