Iron Deficiency Anemia: This condition is the most common cause of conjunctival pallor, malaise, and weakness due to a lack of iron, which is essential for hemoglobin production and oxygen delivery to tissues.

Vitamin Deficiency Anemia (B12 or Folate): Deficiencies in vitamin B12 or folate can lead to anemia, causing symptoms like malaise, weakness, and conjunctival pallor due to impaired red blood cell production.
Chronic Kidney Disease (CKD): CKD can lead to anemia, which presents with malaise, weakness, and conjunctival pallor, among other symptoms, due to the kidney's reduced ability to produce erythropoietin.
Chronic Infections (e.g., Tuberculosis, Endocarditis): Chronic infections can cause prolonged malaise, weakness, and anemia, leading to conjunctival pallor.

Acute Blood Loss: Internal or external bleeding can rapidly lead to anemia, presenting with malaise, weakness, and conjunctival pallor. This condition is life-threatening and requires immediate attention.
Leukemia or Lymphoma: These cancers can cause anemia, leading to the symptoms mentioned, and are critical to diagnose early for effective treatment.
Sickle Cell Disease: Especially in individuals with a known history, sickle cell disease can cause anemia and the associated symptoms due to abnormal hemoglobin.

Myeloproliferative Neoplasms: Rare conditions like polycythemia vera, essential thrombocythemia, or primary myelofibrosis can sometimes present with anemia and the mentioned symptoms, though they are more commonly associated with overproduction of blood cells.
Paroxysmal Nocturnal Hemoglobinuria (PNH): A rare, acquired, life-threatening disease of the blood characterized by the destruction of red blood cells, leading to anemia and potentially the symptoms described.
Congenital Dyserythropoietic Anemias: A group of rare genetic disorders characterized by ineffective erythropoiesis, leading to anemia and its symptoms.