(CNN) Your chest is tight, and at times, it's hard to catch your breath. You wheeze or cough so hard that your sides soon ache.
You're so tired, it's as if all the blood has drained from your body. A fever spikes. You get chills and begin to shake.
These are all signs of pneumonia, a serious lung infection that claims the lives of hundreds of thousands of people around the world each year.
Here are five key things to know about this potentially deadly illness.
Pneumonia can be caused by a virus, bacteria or even a fungus. Although pneumonia itself is not contagious, the germs that can cause it are. If caused by a virus, it can easily develop into bacterial pneumonia, which can be quite serious.
Pneumonia occurs when the air sacs, or alveoli, of the lungs fill with fluid or pus. That makes it harder to take a breath and get enough oxygen. Without treatment, oxygen levels can fall to life-threatening levels.
Pneumonia can occur in one lung or both lungs, which is called double pneumonia. Or you can have it and not even know it, a condition known as "walking pneumonia."
Most healthy people can fight off pneumonia, but for the young, old, frail or immune-compromised, the disease can be tough to battle. In the United States alone, pneumonia kills about 50,000 people a year, mostly adults over 75 and children under 5.
According to UNICEF, more than 2,500 children a day die from pneumonia around the world, most of those under the age of 2, making it the leading cause of death for little ones.
Anyone with a chronic disease such as diabetes, kidney problems, heart failure, HIV/AIDS or a lung disease like COPD is also at high risk, as is anyone undergoing chemotherapy or taking an immunosuppressant drug. Smoking and drinking too much alcohol can also raise your chances of getting the disease.
Many of the symptoms of pneumonia mimic those of a cold or the flu. So how do you tell the difference?
In general, colds tend to come on rather slowly, probably with a runny nose and sore throat. If you add fever, body aches and headache that come on quickly, it could be you have the flu. Pneumonia is usually a complication of cold or flu, when the illness lodges in the lungs.
If your pneumonia is caused by a virus, the symptoms will be flu-like for the first few days: dry cough, fever, headache, shaking chills, extreme fatigue, a poor appetite, and muscle pain and weakness. But then the cough will worsen and produce mucus, fever will spike, and breathing will worsen. You might have a sharp or stabbing chest pain. Lips might turn bluish.
People who have viral pneumonia are at high risk of developing bacterial pneumonia.
With bacterial pneumonia, you could also have a very high fever (105 degrees) and profuse sweating, with fast, labored breathing and a higher pulse. Due to lack of oxygen, there could be a bluish tinge under your nails. There could be mental confusion, especially in the elderly.
If you think you have symptoms of pneumonia, don't hesitate. See a doctor immediately.
To verify that you have pneumonia, your doctor will probably order a chest X-ray, in which the fluid-filled sacs can be clearly seen.
Treatment depends on the cause. For viral pneumonia, an antiviral medication may be prescribed, while antibiotics are used to treat fungal and bacterial pneumonia. Most bacterial pneumonia is caused by Streptococcus bacteria, followed by Haemophilus and staphylococcus bacteria.
Unfortunately, the rise of antibiotic-resistant strains, or serotypes, of Streptococcus bacteria is making it more difficult to treat pneumonia, especially if it is caught in a hospital setting, where resistant bacteria are more commonly found.
The best way to prevent pneumonia is to take advantage of vaccinations. Pneumonia often follows the flu, so getting a yearly flu vaccination is key.
For those in high-risk populations, three types of pneumonia vaccines are currently available: PCV13 or pneumococcal conjugate vaccine, which protects against a few serotypes of Streptococal bacteria; PPSV23 or pneumococcal polysaccharide vaccine, which protects against many more; and Hib, or Haemophilus influenzae type b vaccine.
Each has certain risks and recommendations, so check with your health care provider to be sure the vaccine is safe for you or your family.