An autoimmune disease is defined as

• A. A disease in which the immune system attacks certain cells, tissues, or organs of the body
• B. A disease caused by harmful bacteria
• C. A disease transmitted by vectors
• D. A genetic condition passed down through families

Answer: (A)

An autoimmune disease happens when the immune system, which normally protects you from infections, mistakenly attacks the body's own cells, tissues, or organs. This happens because the system loses tolerance to self-antigens and mounts an immune response against the body's own components, leading to inflammation and damage wherever the attack occurs. That's why the correct description is that the immune system attacks certain cells, tissues, or organs of the body. The other options describe infections from bacteria, diseases spread by vectors, or inherited genetic conditions, none of which capture the idea of the immune system turning its attack inward at the body's own tissues.