Mandela's lung infection could be pneumonia. (Image credit: Media24/Gallo Images/Getty Images)

Nelson Mandela is back in a South African hospital today battling another lung infection, a term often used synonymously with pneumonia.

The health scare comes roughly three months after the former South African president spent 18 days hospitalized with a lung infection and gallstones.

Elderly people are at an increased risk for infections in general, more so if they have many chronic medical problems. As people age, their immune systems are less capable of fighting off infections. Mandela is 94.

As people become more and more infirm, they have a decreased cough response and may aspirate oral secretions into their lungs, raising the risk of pneumonia. And if a person is bedridden, his breathing becomes shallower, raising the risk even more.

The elderly sometimes respond differently to pneumonia; they might lack common symptoms like fever and cough, and instead show signs of confusion. The evaluation of change in physical or mental condition in someone of Mandela's age is broad with much testing needed to make a diagnosis.

There are different kinds of pneumonia, including the viral form, sometimes caused by influenza, the bacterial form, sometimes caused by pneumococcus or tuberculosis, as well as fungal and parasitic forms. Viral or bacterial pneumonias are most common. Bacterial pneumonias or those without a definitive cause are treated with antibiotics and respiratory support.

While pneumonia is a leading infectious cause of death in the elderly, with proper treatment, many people do recover.