Hello. Breathlessness on walking may be caused by either a lung condition or a heart condition. That due to heart conditions develops slowly over days to months together and is also associated with other symptoms like palpitations, swelling of the feet and legs, breathlessness on lying down and fatigue. This is caused by heart failure resulting from any cause like anemia, valvular diseases, blockages of the blood vessels to the heart, hypertension, diabetes, etc. Breathlessness due to any lung condition most commonly is as a result of very severe pneumonia I.e. infection of the lung. It is generally accompanied by very high fever with chills, cough with sputum and fast breathing. Other lung conditions which may cause that are an exacerbation of long standing airway disease like chronic obstructive pulmonary disease or bronchial asthma. If you do experience any of the above symptoms you need to visit your physician as early as possible as early treatment benefits greatly in preventing unnecessary and dangerous complications. Wishing you a speedy recover. Regards, Dr Geeta