Revelation 16:1-2:

And I heard a great voice out of the temple saying to the seven angels, Go your ways, and pour out the vials of the wrath of God upon the earth.
And the first went, and poured out his vial upon the earth; and there fell a noisome and grievous sore upon the men which had the mark of the beast, and upon them which worshipped his image.

Notes on Revelation 16:1-2:

As we see so often, sickness is the deserved punishment on those who disobey God, and those who explicitly reject God by receiving the mark of the beast will definitely be in disobedience.  Note that sickness will NOT fall on those who obey God!  Sickness is a punishment, never a blessing given to the righteous.

See also:

Notes on Romans 2:4
Sickness as Chastening and Judgment in the New Testament
Who or What Causes Sickness?
Does Satan Literally Invent All Diseases?
Condition: Sores