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Singer James Brown died of congestive heart failure on Monday, a day after being admitted to a Georgia hospital with pneumonia. President Gerald Ford died the day after; he had battled pneumonia earlier this year, and his health then rapidly declined. What makes pneumonia so dangerous for old people?

It makes them more vulnerable to other problems. Because of its rapid onset and ability to spread to other parts of the body (as opposed to a stroke or heart attack), pneumonia can be lethal. Even a milder case deals a severe blow to your immune system, which can turn an underlying condition like heart disease into a life-threatening malady. Since older people often have these additional problems, any sort of pneumonia can have dangerous complications. James Brown’s cause of death was officially listed as congestive heart failure, but his pneumonia had likelyweakened his immune system and either aggravated an underlying condition or caused his heart to fail on its own. In general, the more health problems someone has, the more likely it is that a case of pneumonia will turn out to be deadly.

The elderly are also less likely to notice they have pneumonia until it’s too late. Younger patients will visit their doctor with symptoms like chills, shortness of breath, and chest pain, but elderly sufferers are often asymptomatic. This is because their immune response is already in a somewhat weakened state. For instance, younger people cough up sputum when congested, the body’s natural way of clearing out the lungs. (It’s unpleasant, but it is also a healthy response.) People tend to lose lung capacity as they age, which makes it harder for them to cough productively. As a result, they might build up a large amount of sputum without becoming symptomatic. Similarly, elderly patients are less likely to notice the symptoms they do have, since they’re so used to feeling ill.

When left untreated, pneumonia is deadly; it’s considered by the medical community to be as serious as a heart attack. After pus forms in the alveoli, it can spread to the bloodstream, the pleural cavity, or into implanted medical devices, such as a replaced valve or pacemaker.

Even if doctors do spot the disease in an elderly patient, it’s often difficult to administer the necessary antibiotics. A younger person might be cured by taking azithromycin for seven to 10 days. But the ability of the kidneys and the liver to metabolize medications changes as we age: Older people are more susceptible to stomach upset and more sensitive to dosage. Many elderly people are also on a cocktail of medications for their other ailments, further vexing prescription.

Why do the elderly get the disease so often in the first place? They’re commonly undernourished; without the proper nutrients, they have a harder time warding off illness. And even the most pristine nursing homes can be playgrounds for germs, with little access to fresh air. Combine stagnant air with weakened immune systems, and pneumonia is quick to spread.

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Explainer thanksRobert Schreiber of Hebrew Senior Life and Carol Kaplun of Iona Senior Services.