In autoimmune diseases, the primary issue is that the immune system attacks healthy cells. This misdirected immune response occurs when the body's immune system mistakenly identifies its own tissues as foreign threats, leading to inflammation and damage to various organs and systems.
The other options you provided either describe conditions unrelated to autoimmune diseases or mischaracterize the immune response. For example, while viral infections can impact the immune system, they do not define autoimmune diseases. Similarly, in autoimmune diseases, the immune system does not correctly identify foreign invaders; rather, it attacks the body's own cells instead.
So the correct response is: The immune system attacks healthy cells.