According to the University of Rochester Medical Center, people who have a weakened immune system become more susceptible to illness and have trouble fighting these illnesses when they occur. Weak immune systems allow bacteria, viruses and other diseases to run through the body unchecked.
The University of Rochester Medical Center notes that many factors contribute to weakened immune systems. Some people simply acquire weak immune systems at birth. Other medical conditions, such as allergic rhinitis, asthma and eczema, show signs of an overactive immune system, which weakens the immune system over time. Autoimmune diseases like lupus, rheumatoid arthritis and type 2 diabetes usually indicate a weak immune system. Sometimes specific diseases weaken the immune system even after they leave the body. Stress and lack of sleep weaken the immune system, as do a poor diet, smoking and excessive consumption of alcohol. Some medications weaken the immune system, while lack of exercise drains the defenses. Dehydration may also lead to a weaker immune system.
The Centers for Disease Control and Prevention notes that people with weakened immune systems become more susceptible to infections such as mold infections. They may also develop lung and sinus infections and usually have a harder time recovering through treatments.