Penicillin, erythromycin, and tetracycline are among the most useful. Despite this miracle group of drugs pneumonia remains a very important problem. In 1936, before the advent of antibiotics, pneumonia was the number one cause of death in the United States. Most pneumonia fatalities occur in elderly or debilitated patients, or in those who have compromised body defenses as may occur from chronic lung disease, cancer, or after kidney transplantation. However, pneumonia can also be fatal in young adults, particularly when due to organisms such as viruses that do not respond to antibiotics.
Pneumonia is caused by microscopic and submicroscopic organisms that infect the lung tissue.
These are the relatively large organisms that can usually be seen under the microscope. They share features of both, and microbiologists do not all agree on their classification. We now know that pneumonia of any particular cause can have either typical or atypical features, so the terminology is not very helpful. They live only in cells and do not respond to antibiotics. Viral infections are difficult to diagnose in the early stages, in part because the organisms cannot be seen with an ordinary microscope.