Devil was said to send diseases to test someones faith (to God), as well as God.

The church backing up Galen's theory of the perfect design

Sin was dangerous, so committing one would cause illness to be sent as a punishment from God.

Disease was also sent to cleanse someone's soul of sin, as well as punishment for committing a sin

Leprosy was described as one of the main punishments for sin.

Having Leprosy meant you were banished from your community and often had to go to a leper house. You had to wear a cloak and have a bell to allow people to acknowledge your presence.

Humours being out of balance, meaning you had too much of one certain humour (i.e too much yellow bile).