Is Pneumonia Contagious?

Because pneumonia is caused mainly by infectious microbes, pneumonia can be contagious. Pneumonia caused by chemical fumes or other poisons not made by infectious agents is not contagious.

Pneumonia is inflammation of the lung tissue usually but not exclusively caused by infection from bacteria, virus, or fungus. Pneumonia causes the air sacs at the end of the airways in the lungs to fill with pus. If inflammation affects both lungs, the infection is termed double pneumonia. If it affects one lung, it is termed single pneumonia. If it affects only a certain lobe of a lung, it's termed lobar pneumonia. Most pneumonia is caused by bacteria and viruses, but some pneumonia is caused by inhaling toxic chemicals that damage lung tissue. Pneumonia can cause fever, chills, cough, and difficulty breathing. Severe pneumonia can result in death.

Because pneumonia is caused mainly by infectious microbes, pneumonia can be contagious. Pneumonia caused by chemical fumes or other poisons not made by infectious agents is not contagious.

Many contagious pneumonias have names, such as bacterial pneumonia, viral pneumonia, Mycoplasma pneumonia, and MRSA (methicillin-resistant Staphylococcus aureus) pneumonia, that indicate the type of pathogen infecting the lung. Some pneumonias have names that are not as clear (for example, "walking pneumonia," which indicates a milder form of Mycoplasma pneumonia). There are many other descriptive terms, such as community-acquired pneumonia, hospital-acquired pneumonia, and aspiration pneumonia (examples that suggest the source of the organism[s] causing the pneumonia). They are all potentially contagious but not as easily contagious as the flu or COVID-19, for example.

Pneumonia is contagious when the causative pathogens (usually bacteria or viruses) are expelled by an infected person by coughing out infected droplets. These expelled droplets contain the bacteria or virus that causes the pneumonia. These droplets contaminate the mouth or breathing tract of another individual to eventually infect their lungs.

The approximate time when pneumonia becomes contagious varies with the type of infecting agent and may range from one to two days to weeks. In addition, some pneumonias are more highly contagious than others. For example, Mycobacterium and Mycoplasma organisms are highly contagious, but other types, including pneumococcal pneumonia, require optimal conditions to spread to another person and are weakly contagious.

The incubation period for pneumonia depends on the type of pathogen causing the disease, the person's age, and his or her overall health. In general, most pneumonias begin with symptoms similar to those of a cold or the flu that last longer than the flu (about seven to 10 days) and become more severe. The signs and symptoms of pneumonia include

• fever and chills;
• a productive cough (phlegm is produced);
• pains or aches the chest when breathing in or during coughing;
• fatigue;
• shortness of breath;
• nausea and vomiting; and
• diarrhea.

These symptoms may occur as early as one to three days or about a week after "flu-like symptoms" develop.

As described above, pneumonia is caused by infectious agents that can spread to others depending upon the type of organism causing the pneumonia. Usually, the organisms spread person to person by contact with an infected person's mouth or when small droplets that become airborne from coughing or sneezing. In addition, once pneumonia develops in the lungs, it may spread to other lobes of the lung, or even to the other lung. In severe cases, the organisms causing pneumonia may spread to other organs of the body and cause damage or even death.

In general, many bacterial pneumonias are much less contagious after antibiotics have been administered for about 24-48 hours. However, this time period may vary for some organisms, like those that cause tuberculosis. For tuberculosis, it can take two weeks or more on antibiotics before the person is no longer contagious. For viral pneumonias, the person becomes less contagious after the symptoms have begun to recede, especially fever. This may occur about one to two days after someone has no fever, but one may still shed some infectious virus for a week or so with some types of viral pneumonias. It is not unusual to have coughing occasionally for some time (days to weeks) even after someone is not contagious.

If someone has had fever and a cough (especially a productive cough with yellow, green, brown sputum) after having "flu-like symptoms," he or she should contact a medical caregiver. If someone develops any shortness of breath, chest pain, confusion, and/or high fevers, he or she should go to an emergency department. In addition, if someone has a chronic health problem like diabetes, HIV, or other problems that result in a depressed immune system, he or she should see a physician immediately or go to an emergency department if even mild pneumonia symptoms develop.