Immunocompromised people are usually susceptible to colds and flu when there is a sudden drop in temperature, crowded or confined environments, and exertion. People with low immunity are less able to fight off illnesses and are more prone to colds. Stress at work and irregular work and diet can cause low immunity. Elderly people and infants generally have lower immunity. Patients with AIDS have low immunity because their immune system has been damaged. Patients with chronic diseases, such as chronic obstructive pulmonary disease and diabetes, also have lowered immunity. People with low immunity are less able to fight off diseases and are more likely to get colds. The incidence of colds can also increase when people with low immunity are exposed to cold air, or when they are in crowded or confined environments. There are many reasons why people are susceptible to colds. It is recommended to have balanced nutrition, improve bad habits, and enhance the body’s immunity. In case of severe colds, it is recommended to go to the hospital in time.