Explanation

The Prophet (may Allah's peace and blessings be upon him) informs that transitive disease in which the people of the pre-Islamic era of ignorance used to believe, thinking the disease is transmitted by itself to others apart from Allah's decree, is false. And the Tiyarah, which is regarding something audible or visible, as an evil omen whether it is from birds, animals, people with deformities, numbers, days, etc. He mentioned the "Tayr" (bird) specifically because it was commonly known during the pre-Islamic era of ignorance, as a bird would be urged to fly when someone intended to do something like travel, trade, or anything else. If the bird flew to the right, he would take this as a good omen and proceed with what he intended, whereas if the bird flew to the left, he would take this as a bad omen and refrain from doing what he intended. Then, he (may Allah's peace and blessings be upon him) informed that he liked the good omen, which is the happiness and joy that one feels when hearing a good word that makes him have good expectations of his Lord.