Lower respiratory (tract) infection or LTRI is a generic term for an acute infection of the trachea (windpipe), airways and lungs, which make up the lower respiratory system.LTRIs include bronchitis, croup and pneumonia.Although influenza (the flu) can involve the lower respiratory system, it is not considered among the group of illnesses referred to by the term lower respiratory infection..Lower respiratory infections are more common in the winter because people tend to stay indoors and share the same recirculated air (and germs).lthough influenza (the flu) can involve the lower respiratory system, it is not considered among the group of illnesses referred to by the term lower respiratory infection.. Lower respiratory infections are more common in the winter because people tend to stay indoors and share the same recirculated air (and germs).