Influenza: This is the most likely diagnosis given the symptoms of high-grade fever, chills, rigors, coryza (runny nose), and cough, which are classic for influenza, especially during flu season. The sudden onset of these symptoms further supports this diagnosis.

Pneumonia: Could be bacterial or viral, presenting with similar symptoms including fever, chills, cough, and possibly rigors. The presence of coryza might lean more towards a viral etiology, but pneumonia should be considered, especially if there are findings on lung examination or imaging.
Acute Bronchitis: Often presents with cough and may have associated fever, chills, and rigors, especially if caused by a viral or atypical bacterial pathogen.
Common Cold (Rhino-Viral Infection): While typically milder, some cases can present with significant systemic symptoms including fever, chills, and cough, especially in the early phase.

Sepsis: Although less likely given the initial presentation, sepsis can present with fever, chills, and rigors. It's crucial to consider, especially if the patient's condition deteriorates or if there are risk factors for sepsis (e.g., immunocompromised state, recent travel, or exposure to certain pathogens).
Meningitis: Can present with fever, chills, and cough if there is associated pneumonia, but typically includes headache, stiff neck, and altered mental status. It's a critical diagnosis not to miss due to its high morbidity and mortality if untreated.
Tuberculosis (TB): Especially in endemic areas or in individuals with risk factors (e.g., immunocompromised, recent exposure). TB can present with chronic cough, fever, night sweats, and weight loss, but can also have an acute presentation.

Q Fever: Caused by Coxiella burnetii, can present with flu-like symptoms including fever, chills, and cough, but is less common and typically associated with exposure to livestock.
Psittacosis: Caused by Chlamydophila psittaci, transmitted from birds, can present with fever, chills, cough, and headache, but is relatively rare.
Histoplasmosis: A fungal infection that can cause flu-like symptoms, including fever, chills, and cough, especially in immunocompromised individuals or those exposed to bird or bat droppings in endemic areas.

Diagnosis would typically involve:

Clinical assessment and history taking
Laboratory tests such as complete blood count (CBC), blood cultures, and possibly influenza testing (rapid antigen test or PCR) depending on the clinical scenario and epidemiological context.
Imaging studies like chest X-ray if pneumonia is suspected.

Supportive Care: For influenza and other viral infections, focusing on hydration, antipyretics for fever, and rest.
Antiviral Medications: For influenza, especially if started within 48 hours of symptom onset.
Antibiotics: If bacterial pneumonia or another bacterial infection is diagnosed.
Hospitalization: May be necessary for severe cases, especially if there are signs of respiratory distress, failure, or sepsis.