جامع الترمذي

'Ibn Mas'ud narrated: "The Messenger of Allah (s.a.w) stood among us and said: 'One thing does not infect another.' So a Bedouin said: 'O Messenger of Allah! If a camel gets mangy glands and we leave it at the resting place of camels, then all of the camels get mange?' The Messenger of Allah (s.a.w) said: 'Who caused the first to get mange? There is no 'Adwa nor safar. Allah created every soul, so he wrote its life, its provision, and its afflictions.'"

Note: The majority of scholars interpret this to mean that these things in and of themselves do not transmit or cause harm through supernatural or hidden means but that Allah is ultimately in control and any fearful superstition around these is false.