Ibn Khatima was the first to observe that mankind is surrounded by minute bodies which enter the human body i.e. aware of the theory of germs. Ibn Khatima was another great thinker from the Islamic world. During his time, he had seen the ravages of the plague around the Mediterranean. He did much of his work on the transmission of the disease the plague and preventive measures. He wrote a book called "on the Plague" which elaborates on causes of the plague.
It was western Arabism that provided the world with the most concrete affirmation of the doctrine of the contagious character of disease. The cause of that discovery was the same plague that ravaged the world in the 14th century, causing the Black Death. The plague traveled from India to China, Russia, Syria and finally struck the Europe. Many accounts record that nearly half of the population of western Europe suffered from the "Black Death" caused by the plague.
It was two Moorish doctors from Granada; one of them was Ibn-Khatima, who found out that the “Black Death” was caused by contagion carried by rats and fleas.
Ibn-Khatima’s work in original Arabic is still preserved in Escurial. He argued against the prevalent and popular belief that contagion wasn’t the wrath of God. It apparently required courage to defy the popular religious beliefs prevalent in both Europe and Asia which associated the contagion with God’s wrath.