Mal 3:10 – Bring ye all the tithes into the storehouse, that there may be meat in mine house, and prove me now herewith, saith the LORD of hosts, if I will not open you the windows of heaven, and pour you out of blessing, that there shall not be room enough to receive it.
3:11 – And I will rebuke the devourer for your sakes, and he shall not destroy the fruits of your ground; neither shall your vine cast her fruit before the time in the field, saith the LORD of hosts.

Sickness is a devourer. When you are faithful to God with your tithe He will not permit sickness to come near your house.