There are two causes of this sickness, one external and one internal.773 The passive cause of the disease is: the member takes the sickness to itself by reason of its own weakness, or through the malice of a cold, evil complexion, or through excessive heat, caused by movement774 or by some external cause that heats the member. As Galen says in the second book of Prognostica, it is easy (easier) for the humours to move and flow to members that are moved and heated (?).775