Shortness of breath that starts suddenly can be caused by:

• Coronavirus 2019 – This is an infection caused by a virus called SARS-CoV-2.
• Lung problems, such as asthma, infections, or blood clots – These conditions might cause other symptoms, too. For example, a lung infection usually causes a fever and cough.
• Heart problems, such as a heart attack or heart failure – A heart attack can also cause chest pain or pressure. Heart failure is when the heart does not pump as well as it should.
• A severe allergic reaction, called "anaphylaxis"
• Pregnancy

For shortness of breath that happens over weeks to months, the most common causes are:

• Lung problems, such as asthma or chronic obstructive pulmonary disease (COPD) – Both of these conditions can make it hard to breathe. COPD usually happens in people who have smoked for at least several years.
• Heart problems, such as heart failure or a change in the size and shape of the heart (called cardiomyopathy)
• Being overweight or out of shape
• Anxiety or panic disorders