People with a weakened immune system are more likely to get infections and have severe symptoms. They may be more susceptible to pneumonia and other illnesses. Bacteria and viruses, particularly the virus that causes the illness COVID-19, can be lethal to someone with a weakened immune system. People with a weak immune system include those who have cancer, are taking immunosuppressive drugs, have HIV/AIDS, or have some other condition that makes them vulnerable to infection. Additionally, people who are elderly or have another underlying health problem are at greater risk of having a serious reaction to COVID-19. In general, people with a weak immune system are more likely to experience complications from an infection. The best way to prevent problems is by keeping your immune system strong by getting enough rest and physical activity, eating a healthy diet, not using alcohol or drugs, and managing any existing health issues. In addition to being sick with COVID-19, people with weak immune systems are at risk for other problems. For example, they are more likely to develop serious complications from influenza or other illnesses. People who are aging or have another medical issue also are at risk for dehydration during diarrhea outbreaks. The first step in treating someone with a weak immune system is to ensure their immediate need for food, water, and shelter are met.