Why did Islam permit polygyny up to four wives, even though it might transmit viruses and bacteria?

Firstly: One must believe that what Allah has legislated carries no pure or predominant harm; rather, its benefit is pure or its welfare is preponderant. For Allah commands nothing but righteousness, and everything He has commanded has a preponderant welfare and benefit.

Secondly: Diseases that may occur between spouses have no connection to polygyny; they can happen with a single wife. Their cause is rather the transgression of Allah’s law, such as unlawful intercourse, or neglecting purity and cleanliness. These diseases are not known in pure societies but spread among those who violate Allah’s law and deviate, serving as a punishment for the disobedient.