The common causes of pain inside the left side of the rib cage are seen in the following: First, there are diseases of the left side of the chest wall, such as trauma to the left side of the chest wall, chronic strain on the left side of the chest wall, and intercostal neuritis caused by viral infection of the intercostal nerves. Left-sided rib fractures and rib cartilage fractures can cause pain inside the left rib cage. Secondly, diseases of the organs in the left side of the chest cavity are mainly seen in pneumonia, tuberculosis, pleurisy, bronchitis, etc. These diseases are caused by local infection, resulting in the pulling of the pleura when breathing and the pulling of the sensory nerves inside the lung, resulting in painful symptoms. Third, it can be seen in heart diseases, such as myocardial ischemia, angina pectoris, coronary artery disease, and also the corresponding symptoms of chest pain on the left side of the rib cage. Fourthly, abdominal organs and left upper abdominal organs can also show involvement pain under the thoracic ribs due to diseases, such as gastric ulcer, gastritis, etc.