How Contagious Is Pneumonia?

I’m trying to find out if pneumonia is contagious. I have found everything else about this illness except if one can catch it from someone who has it. Any help you can give would be greatly appreciated.

Pneumonia is a common diagnosis, but there are actually many different types of pneumonia, and to answer your question today would require knowing precisely what kind of pneumonia you are referring to. But since this is such a common concern, I will try to describe pneumonia in general, as well as some of the more common forms of the disease and the few that require isolation.

Pneumonia means inflammation of the lung with consolidation. The consolidation refers to fluid or other materials which build up in the air spaces of the lungs, the alveoli, and cause the affected part of the lung to appear white on a standard x-ray. Although doctors can diagnose pneumonia purely from the history and physical exam, that simple exam is not very accurate — in part, because the findings can be minimal or confusing; and, in part because, even with modern diagnostic tests, particularly x-rays, doctors simply aren’t as good at physical diagnosis as they used to be before they could rely on those tests. Therefore, I am very suspicious when someone tells me that they had pneumonia but didn’t have an x-ray. This is particularly true of the diagnosis “walking pneumonia” which — although commonly used — has no real medical meaning.

Pneumonia can be caused by all types of infectious agents, bacterial, viral, fungal, and others; so knowing the precise infectious agent is important. Other important factors include where the pneumonia is caught, whether at home or when one is already in the hospital; whether there are other cases of pneumonia in the community that have a common source; whether there is another disease going around, such as the flu; whether the patient has an underlying condition such as alcoholism or AIDS; and whether the patient is exposed to any particular dusts or aerosols.

Pneumonias caused by the bacterium Streptococcus pneumoniae, the cause of the old ‘lobar pneumonia’ are not considered contagious, since S. pneumoniae is a ubiquitous organism, present throughout the environment and in many healthy people. One doesn’t catch lobar pneumonia by being around someone who has it, and such patients do not need to be isolated. Similarly, other types of pneumonia — those caused by Staph. aureus, common in sick hospitalized patients; pneumonia caused by pneumocystis carinae, seen in people with AIDS or other severely immune depressed people; or bronchopneumonia, often seen in alcoholics or people immobilized in bed — do not require isolation, since they are all caused by organisms commonly found in the environment and carried by many healthy people.

Pneumonia may be caused by several organisms which used to be common, and for which people were commonly isolated; for example, the pneumonia due to measles. Isolation was not useful because almost 100 percent of children would catch measles at some point; and whether a particular case developed into pneumonia had nothing to do with how one caught it. The same is true of chickenpox and influenza pneumonias.

Nonetheless, there are some very rare pneumonias which require strict isolation. The pneumonia of plague is extremely contagious, as is that caused by anthrax and the Ebola virus. All of these can produce a highly fatal pneumonia and people exposed to them are at high risk. Pneumonia caused by Mycobacterium tuberculosis is also transmissible by cough droplets, and patients with this form of tuberculosis must be isolated until they are no longer infectious. Adequate ventilation systems and masks will greatly reduce the risk of other people catching tuberculosis from someone with this form of pneumonia.

Therefore, if you are referring to someone who developed pneumonia in the hospital, or under ordinary circumstances at home, following a cold for example, then I would say that they are not contagious, and no special precautions need to be taken by someone in contact with them. If a person developed fever and swollen glands which progressed to pneumonia after hunting rabbits in the Southwestern U.S. where plague is endemic in small animals, then prompt medical care and careful isolation would be important to prevent the spread of this devastating illness. Although we no longer think much about plague, a recent outbreak of plague pneumonia in India which killed many people shows that this is a disease that can still cause epidemics with many fatalities. However, this is a rare disease at this time in the United States, and so I can safely say that at least 99 percent of pneumonias are not contagious to others, and require no isolation.


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