Relatively vulnerable, the lungs are frequently a site of infection and diseases, such as emphysema, cancer, tuberculosis, and pneumonia. Asthma, however, is perhaps the most commonly incurred lung condition, currently affecting approximately five percent of the populations in developed countries. Rates are continuing to rise and speculation about the cause is abundant. Second-hand smoke, air pollution, and crowded cities are the most frequently blamed environmental factors, but some scientists argue that vaccinations may also be suspect because of their interference with the immune system.