Bacteria need at least hours to grow enough cells to cause illness.

4 hours is the correct answer.

Explanation:

Bacteria need at least 4hours to grow enough cells to cause illness.

In 4hours, bacteria numbers get increase and they cause illness or produce toxins that result in bacterial infection.

When bacteria enter the human body it begins to multiply and the cell of the body gets damaged because of the infection caused by bacteria, that result in illness.