Nelson Mandela is being treated for a lung infection, a term often used synonymously with pneumonia.
Elderly people are at an increased risk for infections in general - more so if the person has many chronic medical problems, but as people age their immune systems are less capable of fighting off infections.
South African officials say Mandela's lung infection is "recurring." The former president is 94 years old.
As elderly people become more and more infirm, they have a decreased cough response and may aspirate oral secretions into their lungs, raising the risk of infections. And if someone is bedridden, their breaths become more shallow, raising the risk even more.
It may seem surprising that it took so long for Mandela's diagnosis to be made public. However, it's possible that it took this long to make a diagnosis.
Elderly people respond differently to pneumonia, meaning 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 types of pneumonia including viral (caused by influenza), bacterial (caused by pneumococcus or tuberculosis), fungal and parasitic. I suspect Mr. Mandela most likely has a viral or bacterial pneumonia. If he does, they are likely treating him with antibiotics and providing respiratory support.
Pneumonia is a leading infectious cause of death in the elderly. But with proper treatment, many do recover.