Viral Pharyngitis: The initial sore throat that improved and then progressed to full-blown cold symptoms (cough, congestion, mild sore throat) is consistent with a viral upper respiratory infection. The negative tests for strep, COVID-19, and flu further support this diagnosis, as these are common causes of sore throat and respiratory symptoms that have been ruled out.

Coronavirus (other than COVID-19): Given the symptoms of a cold and the negative COVID-19 test, other coronaviruses could be responsible for the symptoms.
Rhinovirus Infection: This is a common cause of the common cold, which aligns with the symptoms presented.
Adenovirus Infection: Adenoviruses can cause a range of respiratory symptoms, including sore throat, cough, and congestion.

Epiglottitis: Although rare, epiglottitis is a potentially life-threatening condition that requires immediate medical attention. It can present with sore throat and respiratory symptoms, and a high index of suspicion is necessary, especially if the patient shows signs of severe respiratory distress.
Lemierre's Syndrome: This is a rare but serious condition that can follow a throat infection, characterized by thrombophlebitis of the internal jugular vein and septicemia. It's crucial to consider in patients with persistent or severe throat symptoms.
Mononucleosis (Mono): Caused by Epstein-Barr virus (EBV), mono can present with sore throat, fever, and fatigue. Although not typically life-threatening, it can lead to complications like splenic rupture if not recognized.

Tuberculosis (TB): Although TB is rare in many parts of the world, it can present with chronic cough, sore throat, and other respiratory symptoms. It's essential to consider in patients with risk factors for TB exposure.
Toxoplasmosis: In rare cases, toxoplasmosis can cause respiratory symptoms, although it's more commonly associated with other manifestations. It's particularly relevant in immunocompromised patients or those with specific exposure risks.