Pulmonary Embolism (PE): This is often considered the single most likely diagnosis for sudden onset dyspnea due to its potential for rapid progression and high mortality if untreated. Symptoms can include sudden onset of dyspnea, chest pain, and cough, among others.

Acute Asthma Exacerbation: Sudden onset dyspnea can be a hallmark of an asthma attack, especially if triggered by allergens or respiratory irritants. Wheezing, cough, and chest tightness are common accompanying symptoms.
Chronic Obstructive Pulmonary Disease (COPD) Exacerbation: Similar to asthma, COPD exacerbations can present with sudden worsening of dyspnea, cough, and sputum production, often triggered by infections or air pollutants.
Pneumonia: Community-acquired or hospital-acquired pneumonia can cause sudden onset dyspnea, especially in vulnerable populations such as the elderly or those with compromised immune systems. Fever, cough, and chest pain are typical accompanying symptoms.
Acute Coronary Syndrome (ACS): While more commonly associated with chest pain, ACS (including myocardial infarction) can present with sudden onset dyspnea, especially in diabetic patients or those with atypical presentations.

Pulmonary Edema: Cardiogenic or non-cardiogenic pulmonary edema can cause sudden and severe dyspnea. Cardiogenic pulmonary edema is due to heart failure, while non-cardiogenic is often due to acute respiratory distress syndrome (ARDS).
Anaphylaxis: A severe, life-threatening allergic reaction that can cause sudden onset dyspnea, along with other symptoms like rash, swelling, and hypotension.
Tension Pneumothorax: A medical emergency where air enters the pleural space and is unable to escape, causing the lung to collapse. Symptoms include sudden onset dyspnea, chest pain, and decreased breath sounds on the affected side.

Foreign Body Aspiration: More common in children, but can occur in adults, especially those with altered mental status. Sudden onset dyspnea can be accompanied by choking or coughing.
Pulmonary Venous Thrombosis: A rare condition where thrombosis occurs in the pulmonary veins, which can mimic pulmonary embolism in presentation.
Amniotic Fluid Embolism: A rare but catastrophic event during pregnancy where amniotic fluid enters the maternal circulation, causing sudden onset dyspnea, hypotension, and potentially cardiac arrest.