Respiratory tract infections (RTIs) can affect any part of your body involved in breathing, from your nose and throat down to your airways and lungs. RTIs are divided into two categories: upper and lower. Adults get between two and three upper RTIs, such as the common cold, per year. Children, especially young children, may get more because their immune system is still developing. Lower RTIs, such as pneumonia, tend to be more serious than upper RTIs.