Pneumonia: This is a common condition that can cause shortness of breath, tachypnea, and is often associated with an infection. The symptoms can range from mild to severe and can be caused by bacteria, viruses, or fungi.

Acute Bronchitis: Often caused by a viral infection, acute bronchitis can lead to shortness of breath and tachypnea, especially if there is significant airway inflammation.
Chronic Obstructive Pulmonary Disease (COPD) Exacerbation: If the patient has a history of COPD, an exacerbation could be triggered by an infection, leading to increased shortness of breath and tachypnea.
Asthma Exacerbation: Similar to COPD, an asthma exacerbation can be triggered by an infection, resulting in symptoms of shortness of breath and tachypnea.

Pulmonary Embolism: Although not directly caused by an infection, pulmonary embolism can present with sudden onset of shortness of breath and tachypnea. It is crucial to consider this diagnosis due to its high mortality rate if untreated.
Sepsis: If the infection is severe and has led to sepsis, the patient could exhibit shortness of breath and tachypnea among other systemic symptoms. Early recognition and treatment of sepsis are critical.
Cardiogenic Pulmonary Edema: Heart failure can lead to fluid accumulation in the lungs, causing shortness of breath and tachypnea. While not directly an infection, it's a critical condition to identify.

Cystic Fibrosis Exacerbation: If the patient has cystic fibrosis, an exacerbation could be triggered by an infection, leading to increased respiratory symptoms.
Pneumocystis jirovecii Pneumonia (PCP): Typically seen in immunocompromised patients, PCP can cause severe pneumonia with symptoms of shortness of breath and tachypnea.
Tuberculosis (TB): Although less common in some regions, TB can cause chronic infection of the lungs, leading to symptoms of shortness of breath and tachypnea, especially if it progresses to a more severe form.