How can the hadith "there is no contagion and no evil omen" be reconciled with the reality of infectious diseases?

Infectious diseases do not transmit by their inherent nature; rather, Allah is the One who makes them a cause for infection, and sometimes this may not occur. The denial of contagion in the Hadith (definition) refers to negating the belief of the pre-Islamic era (Jahiliyyah) that diseases transmit by themselves without Allah's decree. The prohibition against bringing a sick person to a healthy one, the command to flee from a leper, and avoiding places of plague are all part of avoiding the causes that Allah has made a reason for perishing or harm, just like avoiding throwing oneself into water or fire.