How sound is the hadith: “There is no adwa (contagion) and no tiyarah (evil omen),” what does it mean, and how can the apparent contradiction between it and the hadith: “Flee from the leper as you would flee from a lion” be resolved?

The hadith "There is no 'Adwa (transference of disease without Allah's Preordainment) and no Tiyara (evil omens)..." and the hadith "Flee from the leper as you flee from the lion" are both authentic. Scholars have reconciled them by explaining that diseases do not inherently transmit by their nature. Rather, it is Allah Almighty who makes them a cause for infection and transmission.