Viral infection: A viral infection, such as mononucleosis or a viral upper respiratory tract infection, is a common cause of malaise that can last for several weeks. The prolonged duration of symptoms and the presence of malaise make this a likely diagnosis.

Anemia: Anemia, particularly iron-deficiency anemia, can cause malaise due to a decrease in oxygen delivery to tissues. This is a common condition that can be diagnosed with laboratory tests.
Hypothyroidism: Hypothyroidism is a condition where the thyroid gland does not produce enough thyroid hormones, leading to malaise, fatigue, and other symptoms. This condition is relatively common and can be diagnosed with thyroid function tests.
Depression: Depression is a mental health condition that can cause malaise, fatigue, and a lack of interest in activities. This is a common condition that can be diagnosed with a thorough psychiatric evaluation.
Chronic fatigue syndrome: Chronic fatigue syndrome is a condition characterized by persistent fatigue that is not relieved by rest. This condition can be diagnosed based on clinical criteria.

Lymphoma: Lymphoma is a type of cancer that can cause malaise, fatigue, and weight loss. Although this is a less common condition, it is important to consider due to its potential severity and the need for prompt treatment.
Tuberculosis: Tuberculosis is a bacterial infection that can cause malaise, fatigue, and weight loss. This condition is important to consider, particularly in individuals with a history of exposure or immunocompromised status.
HIV infection: HIV infection can cause malaise, fatigue, and weight loss, particularly in the acute phase of the infection. This condition is important to consider due to its potential severity and the need for prompt treatment.
Endocarditis: Endocarditis is a bacterial infection of the heart valves that can cause malaise, fatigue, and fever. This condition is important to consider due to its potential severity and the need for prompt treatment.

Addison's disease: Addison's disease is a rare condition where the adrenal glands do not produce enough cortisol and aldosterone, leading to malaise, fatigue, and weight loss.
Pernicious anemia: Pernicious anemia is a rare condition where the body is unable to absorb vitamin B12, leading to malaise, fatigue, and neurological symptoms.
Systemic lupus erythematosus: Systemic lupus erythematosus is a rare autoimmune condition that can cause malaise, fatigue, and a range of other symptoms.
Sarcoidosis: Sarcoidosis is a rare condition where granulomas form in various organs, leading to malaise, fatigue, and a range of other symptoms.