Infectious diseases of the upper or lower respiratory tracts are known as respiratory tract infections. The upper respiratory system starts at your sinuses and ends at vocal cords and the lower respiratory system starts at vocal cords and ends at your lungs.
Simply, RTIs are any infections that cause trouble in breathing.
Upper respiratory infections include common cold, tonsils, acute rhinitis, acute rhinosinusitis, and acute otitis media. Lower respiratory tract infections include bronchitis, pneumonia, Tracheitis, and bronchiolitis.
People with weaker immune systems, children, and older adults are more prone to these infections.