Yes, pneumonia can be contagious. But here's why it depends on the form.

For the second half of the 19th century and the early part of the 20th, pneumonia was a leading cause of death that killed hundreds of thousands of people. Sir William Osler – usually considered the father of modern medicine – described pneumonia at the time as "the most fatal of all acute diseases" because it killed about one in four of the people it infected during the Civil War. Thanks to advancements in modern medicine, though, pneumonia in the U.S. is not as fatal as it once was. Still, some 41,000 people in the country die from the disease every year – and more than 1.4 million people still visit an emergency room because of it, according to the U.S. Centers for Disease Control and Prevention.

Here's what pneumonia is, what causes it and how the disease is usually treated once it has been diagnosed.  

What is pneumonia?

Pneumonia is a serious respiratory infection that exists inside of the lungs, says Dr. Jillian Diuguid-Gerber, an internal medicine specialist at NYC Health + Hospitals/Woodhull. She explains that it's what happens when the tiny air sacs inside of the lungs (called alveoli) fill with fluid or pus. 

This can lead to clogged airways or inflammation that can make it difficult to breathe. Telltale shortness of breath, rapid breathing or other breathing difficulties are the most worrisome symptoms of pneumonia, as they "can be a sign of low oxygen levels," says Diuguid-Gerber. But breathing difficulties are not the only symptom of the disease.

Other symptoms include chest pain, fatigue, loss of appetite, sweating "and fever and cough are especially common," says Dr. David Levine, pediatrician and the Chief of Ambulatory Pediatrics at the Atlantic Health System’s Goryeb Children’s Hospital in New Jersey.

Symptoms of pneumonia can be concerning in anyone but are especially problematic in children as their lungs are still developing; they have smaller airways than adults. "Pneumonia can also be very worrisome in people with weakened immune systems and with existing lung problems," adds Diuguid-Gerber.

What causes pneumonia? 

Pneumonia is caused by bacteria, viruses or fungi, "but usually when people are thinking of pneumonia, they are thinking of the kind caused by bacteria," says Diuguid-Gerber. She explains that the most common bacteria to cause the disease is called Streptococcus pneumoniae, but other bacteria can also cause it. These include Haemophilus influenzae, Mycoplasma pneumoniae and Staphylococcus aureus.

Beyond bacteria, "viruses are also common causes of pneumonia," says Diuguid-Gerber. These include any virus that can affect the respiratory system, including influenza (flu) viruses, the adenovirus (similar to the common cold), RSV, human metapneumovirus and the coronavirus (COVID-19). 

While bacteria and viruses are behind most pneumonia cases, Levine says that fungi can also cause pneumonia. Pneumocystis jirovecii and Histoplasma capsulatum are the types of fungi most commonly responsible.  

The cause of the disease also determines whether pneumonia is contagious or not. The viral forms of it that are most contagious are transmitted "through respiratory droplets being released into the air when a person coughs, sneezes or even talks," says Levine. 

How is pneumonia treated? 

Regardless of what has caused the disease to develop, symptoms of pneumonia are often targeted through everyday remedies like getting plenty of rest, staying hydrated with plenty of fluids using a humidifier to soothe irritated airways and taking over-the-counter pain relievers like ibuprofen and acetaminophen to treat discomfort. Hospitalization may be needed in order to provide oxygen support or intravenous hydration.

Treating the cause of the disease can often be done with nothing more than "at-home care," says Levine, "but some cases of the disease require antibiotics." Antibiotics only treat bacterial forms of pneumonia, however, and antiviral medications are used to treat viral forms for the disease, Diuguid-Gerber explains. Antifungal medications are usually needed to treat fungal pneumonia.

The discomfort of pneumonia and associated treatment can often be altogether avoided through vaccination. "Getting the pneumonia vaccine, flu vaccine or COVID vaccine could all help prevent pneumonia," says Diuguid-Gerber. Indeed, pneumonia vaccines have been shown to be between 45% and 75% effective in preventing the disease, depending on what type of pneumonia you're dealing with.

And even if you're among the people who get pneumonia despite having gotten the vaccine, Levine adds, "vaccines can prevent many of the worst outcomes of the disease."