It is still a serious disease, especially in infants and the elderly, who are most vulnerable. In spite of the advent of antibiotic therapy in the 1940's and a reduction in the mortality rate for all infectious diseases, pneumonia currently accounts for 37 per cent of hospital deaths and still ranks among the ten leading causes of death in the United States. Infectious pneumonia may be caused by bacteria, viruses, fungi, or protozoa. The microorganisms that give rise to pneumonia are often normal flora in the upper respiratory tract. They cause no harm unless resistance is severely lowered by some other factor, such as a severe cold, disease, alcoholism, or general poor health.
Age is also a factor.
Nosocomial pneumonia is common in hospitalized patients. When resistance is lowered or the conditions are favorable, the infectious agents invade the lungs. Lobar Pneumonia . Pneumonia that affects a segment or an entire lobe of the lung is called lobar pneumonia. When both lungs are affected, the disease is called bilateral, or double, pneumonia.