Definition: Condition where the immune system does not function properly.

Explanation: A primary immune deficiency is a condition where the immune system does not function properly. These conditions can be inherited or can occur spontaneously. More than 70 primary immune deficiency disorders are recognised by the World Health Organisation. The disorders range widely in severity.

Primary immune deficiency disorders generally manifest as "ordinary" infections, often of the sinuses, ears, or lungs, or as gastrointestinal problems or inflammations of the joints. Frequently these infections are chronic, and early diagnosis and treatment is essential to preventing the recurring infections from causing permanent damage.

The incidence of primary immune deficiency varies. Some disorders are relatively common, affecting one person in 400, while others are much rarer, affecting less than one person in 250,000.

Source: http://www.answers.com/primary%20immune%20deficiency