Final answer:
Secondary immunodeficiencies, of which malnutrition is the most common cause, can sometimes be reversed because they are due to external factors like diet or stress. Primary immunodeficiencies are usually inherited and are then manifested early in life, resulting in increased susceptibility to infections.
Step-by-step explanation:
The most common cause of secondary immunodeficiencies is malnutrition, which affects both innate and adaptive immunity globally. Secondary immunodeficiencies can sometimes be reversed because they are typically due to external factors, such as nutrition, that when corrected can lead to the restoration of immune system function. In contrast, primary immunodeficiencies are caused by inherited defects and are usually present at birth or become apparent in early childhood. An example of such a primary condition is the chronic granulomatous disease, which causes increased susceptibility to bacterial infections due to a defect in the immune system's ability to destroy bacteria.