What illness is caused by bacteria, viruses, or fungi entering and multiplying within the body?

The correct answer is the term that encompasses the condition when pathogenic microorganisms such as bacteria, viruses, or fungi invade the body, begin to multiply, and lead to various health issues. An infection occurs as these pathogens overcome the body’s defenses, often resulting in symptoms like fever, inflammation, and other signs of illness. This process is marked by the pathogens actively reproducing within the host's tissues, which can trigger an immune response to fight off the invading organisms.

In contrast, inflammation refers specifically to the body’s response to injury or infection, characterized by redness, heat, swelling, and pain. While inflammation can occur during an infection, it does not itself denote the presence or multiplication of pathogens.

An allergy is an immune response to a substance that the body mistakenly identifies as harmful, leading to symptoms that can include sneezing, itching, or hives; it is not caused by bacteria, viruses, or fungi.

An autoimmune disease occurs when the immune system mistakenly attacks the body's own tissues, leading to various chronic health issues. It involves a different mechanism from infections, as it does not involve the entry or multiplication of external pathogens.

Thus, the term that accurately describes the situation where harmful microorganisms proliferate in the body leading to illness is indeed infection