Acute Bronchitis: This is the most likely diagnosis given the symptoms of high fever, fatigue, and phlegm. Acute bronchitis is a common condition characterized by inflammation of the bronchial tubes, often caused by a viral infection, which leads to symptoms such as cough, phlegm production, fever, and fatigue.

Pneumonia: This condition, which involves an infection of the lungs, can present with high fever, fatigue, and production of phlegm. It is more severe than acute bronchitis and can be caused by bacteria, viruses, or fungi.
Influenza: The flu is a viral infection that can cause high fever, fatigue, and respiratory symptoms including cough and phlegm production. It is highly contagious and seasonal.
Acute Sinusitis: This condition involves an infection or inflammation of the sinuses and can present with symptoms such as facial pain, headache, fever, fatigue, and cough, which may produce phlegm.

Tuberculosis (TB): Although less common in many parts of the world, TB is a serious infection that can present with chronic cough, fever, fatigue, and phlegm production. It is crucial to consider TB, especially in individuals with risk factors such as immunocompromised states or recent travel to endemic areas.
Pulmonary Embolism: While less likely to present with phlegm, pulmonary embolism can cause sudden onset of fever, fatigue, and respiratory symptoms. It is a life-threatening condition that requires immediate medical attention.
Sepsis: Sepsis is a life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs. It can present with fever, fatigue, and respiratory symptoms among others, and is critical not to miss due to its high mortality rate.

Cystic Fibrosis: This is a genetic disorder that leads to severe respiratory and digestive problems. While it is rare and typically diagnosed in childhood, it can present with chronic cough, phlegm production, and recurrent infections.
Bronchiectasis: A condition characterized by damage to the airways, leading to chronic cough, phlegm production, and recurrent infections. It is less common and often associated with underlying conditions or risk factors.
Lung Cancer: Although rare as an initial presentation with these symptoms, lung cancer can cause cough, fatigue, and in some cases, fever and phlegm production, especially in advanced stages or in individuals with significant smoking history.