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If a healthy person is near a person that has pneumonia, is there any risk of infection for the former?

If so, how is it passed on? Is it via coughing / sneezing or skin contact?

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Mostly it depends on the cause of the pneumonia.

Pneumonia is the name for infection or inflammation of the lungs. It can be primary or secondary. A good example of the difference is pneumonia caused by the influenza virus.

Influenza affects the lungs directly. That's a primary pneumonia, and yes, this is contagious, as the air one coughs out contains droplets carrying the virus. Pneumonia caused by viruses is more common in children and young adults (except for influenza, which affects all age groups), and is therefore contagious to people who have never been exposed to the virus - usually other children/young adults. In general, primary pneumonia tends to be more contagious, and besides viruses, it can be caused by mycoplasma and tuberculosis. These are contagious.

The damage done by the influenza virus in the lungs leaves the person susceptible to attack by bacteria that would not normally gain a foothold in the lung. This is secondary pneumonia, and because your lungs are not damaged - and you already have the same bacteria in your mouth and throat as someone with secondary pneumonia but are not ill - you are much less likely to get sick from being exposed to bacterial pneumonia than from someone with viral pneumonia.

Similarly, pneumonia is more common in smokers with damaged lungs, or in people with some degree of decreased immunocompetence. Alcohol, other drugs, and people with swallowing disorders can aspirate leading to pneumonia; surgery or chest injuries where deep breathing hurts can also lead to pneumonia. These pneumonias aren't likely to be contagious. Finally, some pneumonias are caused by exposure to chemicals. Since there's no infectious agent involved, you can't be affected.

There are now vaccines against some of the bacterial pneumonias, so vaccines against influenza and others help.

Since you might not know what kind of pneumonia someone has, practicing good hygiene is always a good idea.

Pneumonia Mayo Clinic
Pneumonia

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Pneumonia may be contagious if it is caused by an infectious microbe. But if pneumonia is caused by chemical fumes or other poisons, then it is not contagious.

If treatment for pneumonia is administered early on, then the time pneumonia is contagious for is shorter. For example, a person with bacterial pneumonia will stop being contagious within two days of taking antibiotics. For other types of pneumonia – like the one that can cause tuberculosis – the treatment may have to be administered for at least two weeks before a person is no longer contagious.

On the other hand, individuals with viral pneumonia are less contagious after symptoms have subsided. The key is to reduce your ability to contaminate others by utilizing proper hygiene and avoiding others as best as possible.

Bacterial pneumonia can be caused by different types of bacteria, including streptococcal pneumonia (most common in adults), chlamydophila pneumonia, and H. influenza type B pneumonia (most common in children).

The common signs and symptoms of bacterial pneumonia include high fever, cough with phlegm, chills, chest pain when breathing or coughing, rapid breathing, shortness of breath, and loss of appetite.

Bacterial pneumonia is very contagious, so beginning antibiotic treatment right away can reduce your contagiousness.

Viral pneumonia is caused by a virus and commonly affects children. Viral pneumonia may clear up within three weeks, but does increase the risk of bacterial pneumonia. Symptoms of viral pneumonia are similar to the flu with fever, aches, and cough. Symptoms may worsen within the first two days, then proceed to improve.

Viral pneumonia is highly contagious and can actually spread quicker than bacterial or fungal pneumonia.

There are three subtypes of fungal pneumonia: coccidioides, histoplasma, and cryptococcus. If a person with a weakened immune system inhales a fungus that’s how they can become sick. Symptoms include fever, dry cough, fatigue, and shortness of breath. Unlike other types, fungal pneumonia is not contagious, but symptoms may appear worse in those with weaker immune systems.

Aspiration pneumonia occurs when a person inhales food or other objects into their lungs. Although not contagious, aspiration pneumonia can be life-threatening, so immediate medical attention is required.

Walking pneumonia is a milder form of pneumonia that presents symptoms similar to a cold. Those symptoms include low-grade fever, persistent dry cough, fatigue and tiredness, shortness of breath, chest pain, and loss of appetite. Walking pneumonia is less severe than full-blown pneumonia, yet it can still be transmitted through droplet infection. Steering clear of others can help reduce transmission, along with covering your mouth and nose when coughing or sneezing.

Source: Is pneumonia contagious?

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