The very young and the very old are alike in many ways in terms of health, and you’ll hear the comparison often. That, in fact, is the case with pneumonia. The two groups of people most susceptible to pneumonia are the very young and adults over the age of 65.

What could these two groups possibly have in common? Answer: An immune system that is not up to the task. In short, pneumonia is the scourge of a weakened immune system.

In the very young, the immune system is not fully developed, and in people over 65 the immune system is beginning to fail. Why? Because this older population has more chronic illness, more history of cardiovascular disease, more history of emphysema, more history of diabetes, and is more likely to have been treated with chemotherapy for some sort of cancer than any other segment of the population.

Pneumonia is an inflammation of the lungs that’s usually caused by an infection from bacteria, viruses, funguses, or anything else that can grow in the lungs. There are about 50 different types of pneumonia, but the most common ones are bacterial pneumonias, which usually produce chills, high fever, sweating, chest pain when you breathe, and coughing with thick green or yellow mucus.

Viral pneumonias are more frequent in the wintertime and are very common among people with a long history of cardiovascular lung disease like emphysema. A viral pneumonia will start with a nonproductive cough (one without sputum or phlegm), followed by fever, muscle ache, fatigue, and difficulties breathing.

Pneumonia is not a contagious disease, so you can’t catch it from somebody else, but there are many places to pick up the bacteria, viruses, and fungi that can cause pneumonia. You will be exposed to some of them in the course of your everyday life; these are called community-acquired pneumonias. Elderly people on ventilators or in the intensive care unit of a hospital often get what’s called hospital-acquired pneumonia, because of all the bugs that are prevalent in hospitals.

Some people, as they get older, can lose their gag reflex and get aspiration pneumonia; that’s when some of the contents of the stomach is regurgitated into the back of their throat and falls into their lungs.

Pneumonia can be life-threatening. If you’re 65 or older, and you think you have pneumonia, the earlier you see a doctor, the better. If you are coughing and are experiencing a shortness of breath along with an unexplained fever of 102, seek attention immediately. Because if you have pneumonia and you’re in those upper decades of life and you have other medical problems, you could be dead in twenty-four hours.

A doctor can diagnosis pneumonia by doing a chest X-ray, by checking for bacteria in your bloodstream or in your sputum, or by finding a fluid accumulation in the lungs with a bronchoscope, which is a thin viewing instrument that allows the doctor to examine your airways. If it’s bacterial pneumonia, you’ll be treated with aggressive antibiotics, while viral pneumonias get antiviral medications. Special medications are available if the pneumonia is due to a fungus like mycoplasma.

Plenty of rest and fluids are always recommended for pneumonia. Most severe pneumonias in older patients are usually treated in the hospital. Pneumonia is not something you want to fool around with; it needs to be treated very aggressively.

The best way to prevent pneumonia is to keep your immune system strong. One way to do this is by getting your annual flu shot, which everyone should get but especially those who are over 65. There’s also a pneumonia vaccine that older folks should get, especially those whose immune systems are compromised by some other disease.

The other hallmarks of prevention have to do with a healthy lifestyle—don’t smoke, eat a balanced diet, and get proper rest. Oh, and wash your hands regularly, too. That’s a good rule for everyone, but especially so for the very young and for senior citizens.

Click here to check out Dr. Manny's book The Check List (Harper Collins, 2007), from which this article was excerpted.

Dr. Manny Alvarez is the managing editor of health news at FOXNews.com, and is a regular medical contributor on the FOX News Channel. He is chairman of the Department of Obstetrics and Gynecology and Reproductive Science at Hackensack University Medical Center in New Jersey. Additionally, Alvarez is Adjunct Professor of Obstetrics and Gynecology at New York University School of Medicine in New York City.

Dr. Manny Alvarez serves as Fox News Channel's senior managing health editor. He also serves as chairman of the department of obstetrics/gynecology and reproductive science at Hackensack University Medical Center in New Jersey. Click here for more information on Dr. Manny's work with Hackensack University Medical Center. Visit AskDrManny.com for more.