"When a member of one's household is unwell, one should go to a Rabbi and ask him to pray for mercy" is what Rav Pinchas bar Chama taught us in Bava Basra (116a) based on the verse in Mishlei (16:14):
"The wrath of the King are messengers of death: but a wise man will pacify it."
It's customary to also give charity to the poor, since "Repentance, Prayer and Charity abolish evil decrees".
The custom is to make a special "Mi Sheberach" prayer in the synagogue for the patient; even on Shabbat, if the condition is life threatening.
In extreme cases, the patient's name is changed (usually by adding another name), as this is another effective way of abolishing evil decrees against a person.
Source: Kitzur Shulchan Aruch 192:2
Wednesday, 28 Tishrei 5775