Viral Upper Respiratory Tract Infection (URTI): This is the most common cause of mild sore throat, cough, and congestion. The presence of yellow/gray mucus and mild body ache further supports this diagnosis, as these symptoms are typical of a viral infection.

Acute Bronchitis: Often presents with cough and mucus production, which could be yellow or gray, indicating the presence of an infection. The mild body ache and congestion also align with acute bronchitis.
Allergic Rhinitis: Can cause congestion, cough (especially if postnasal drip is present), and a sore throat. The absence of high fever and the presence of mild symptoms make this a plausible diagnosis.
Influenza: Although typically presents with more severe symptoms, mild cases can occur, especially in individuals who have been vaccinated or have a mild strain of the virus. The body ache and cough are consistent with influenza.

Pneumonia: While less likely given the mild nature of symptoms, pneumonia can present subtly, especially in its early stages or in certain populations (e.g., the elderly or those with compromised immune systems). The cough and mucus production could be indicative of a lower respiratory infection.
Pertussis (Whooping Cough): Can present with a persistent cough and mucus production. Although typically associated with a distinctive whoop, this may not always be present, especially in adults or vaccinated individuals.
Tuberculosis (TB): A chronic infection that can present with cough, mucus production, and systemic symptoms like body aches. Although less common, TB is a critical diagnosis not to miss due to its public health implications and the need for specific treatment.

Lemierre's Syndrome: A rare condition characterized by a sore throat followed by neck pain and high fever, due to a bacterial infection. The mild symptoms described do not strongly suggest this diagnosis, but it remains a rare possibility.
Sarcoidosis: Can present with respiratory symptoms including cough and congestion, though it is much less common and typically associated with other systemic symptoms or findings on imaging studies.
Histoplasmosis: A fungal infection that can cause respiratory symptoms, including cough and mucus production. It is more common in certain geographic areas or in individuals with exposure to bird or bat droppings.