Final answer:
The correct statement about bacteria causing diseases is that we don't know of any harmful archaea. Bacteria can cause serious illnesses, but not all bacteria are harmful to humans, and antibiotics aren't always effective due to antibiotic resistance.
Step-by-step explanation:
Regarding bacteria causing diseases, the true statement is that we don't know of any harmful archaea. Bacteria are responsible for many types of diseases in humans, such as strep throat, tuberculosis, and pneumonia. While some bacteria can be used as biological weapons, not all bacteria are harmful; some have beneficial roles in our bodies and the environment. Antibiotics are used to treat bacterial infections by targeting parts of the bacterial cell, but they do not work against viruses, and not all bacterial infections can be treated with antibiotics due to the development of antibiotic-resistant bacteria.