Viral Infection (e.g., Influenza): This is the most likely diagnosis given the combination of symptoms such as fever, cold, cough, and leg pain, which can be myalgia associated with viral infections.

Pneumonia: Could explain the fever, cough, and if severe, could lead to systemic symptoms including myalgias or leg pain due to hypoxia or severe infection.
Bronchitis: Similar to pneumonia, could explain the respiratory symptoms and potentially the fever and leg pain if the infection is severe or if there's an associated viral component.
Musculoskeletal Injury or Strain: If the leg pain is prominent and localized, it could be due to a musculoskeletal issue, with the fever and respiratory symptoms being coincidental or due to a separate, less severe viral infection.

Deep Vein Thrombosis (DVT): Although less likely, DVT is a critical diagnosis to consider, especially if the leg pain is unilateral and severe. It can be associated with fever and could be a complication of prolonged bed rest due to illness or a separate condition altogether.
Osteomyelitis or Septic Arthritis: Infections of the bone or joint, respectively, which could cause leg pain and fever. These conditions are serious and require prompt treatment.
Pulmonary Embolism: If the patient has risk factors or if the cough and fever are accompanied by severe dyspnea or chest pain, pulmonary embolism must be considered due to its high mortality rate.

Lymphoma or Leukemia: Systemic cancers that could present with a wide range of symptoms including fever, cough (if there's pulmonary involvement), and bone or joint pain due to marrow infiltration.
Endocarditis: An infection of the heart valves, which could cause fever, systemic symptoms, and potentially leg pain if there are embolic phenomena.
Vasculitis: Certain types of vasculitis, like giant cell arteritis or polyarteritis nodosa, could present with systemic symptoms including fever, and if they involve the vessels of the legs, could cause leg pain.