Iin, the case of medieval bio warfare?

There are three types of weapons banned by the UN in wars today, they are considered WMDs, use of either is considered a war crime. They are Nuclear weapons, Chemical weapons, Biological Weapons. Of them Nuclear weapons are of course the deadliest, but they have only been used once , the atomic bombing of Hiroshima and Nagasaki in 1945 in WW2. Chemical weapons were used in their peak in WW1, especially with chlorine gas poisoning and has lead to many thousand of deaths (there are reports of illegal use in Syria in recent times). But the oldest and by far the deadliest of them all is bio weapons, and biological warfare has persisted since the ancient ages and has resulted in millions of deaths, more than chemical weapons and nuclear weapons have ever combined. One of the earliest and confirmed case of bio-warfare was during the Great Plague of the 14th Century. One of the deadliest disease known to mankind is the bubanoic plague, caused by the bacteria Yersinia pestis that was spread through rats. It was highly infectious and at that time there was no cure and catching it meant certain death. The disease is responsible for wiping out nearly 50% of the European population. So how did sptread so much? The period 13th-14th century was known as the dark age because it was entirely filled with war and chaose, of them the most troublesome were the Mongol Conquests. The Mongols not only launched savage genocidal conquests but with the migration comes disease, and en route to battles soldiers became infected with the disease (possible through rat bites, exact cause will never be known) and they spread it over the territories they invaded. They were savages, and when they saw the catastrophe, they would catapult dead bodies of infected people into enemy territories, and the highly contagious disease would spread all over their lands, wiping out entire population of the regions that were struck. Other nations or factions at war learned the technique (of using dead bodies of bubanoic plague infected people as weapons) from the Mongols and used it in their own wars in the rest of Europe, Asia and Africa (the 12th-14th century was filled with wars all around the known parts of the world). These are among the earliest records of biological warfare, and use of biological agents as weapons, and this was by far the deadliest. the Mongol Conquests eventually caused over 40 million deaths (which is a huge figure compared to the low population then), and a vast majority was a result of their biological warfare using the bubanoic plague infected bodies that they threw at enemy territories. But as I said other groups at war also learned and used the technique. The highest estimate of death toll from the 14th C Great Plague Epidemic combining Europe, Asia and Africa is 200 million out of a total world population of 450 million that time, and in Europe 50 million/total population of 80 million. Since then bio weapons were used now and then, but never to the extent they were in the 14th century, but results have always been catastrophic. Later generations of invaders learned that bio agents are uncontrollable and risked infecting their own kind (very few in history had the bravery, determination and savagery the Mongols did) and so would gradually stop using them. Today use of bio weapons is completely prohibited and a war crime, not to mention even the cruelest generals today are so civilized they would never think about using such destructive substance, the bubanoic plague which is responsible for the deadliest epidemic in history is today a curable with a vaccine. But there are numerous new deadly incurable diseases today, and with how sophisticated science and medicine has become they could be easily contained and launched in modern wars without harming the attackers. But such will never be allowed in the civilized world of the 21st century. But is it normal that moral values were so low in the past, people were so cruel and such destructive weapons were used in wars of the past?