From The Annotated Luther, Volume 4: Pastoral Writings, page 404
“You ought to think this way:
‘Very well, by God’s decree the enemy has sent us poison and deadly refuse.
Therefore I shall ask God mercifully to protect us.
Then I shall fumigate, help purify the air, administer medicine, and take it.
I shall avoid places and persons where my presence is not needed in order not to become contaminated and thus perchance infect and pollute others, and so cause their death as a result of my negligence.
If God should wish to take me, He will surely find me, and I have done what He has expected of me, and so I am not responsible for either my own death or the death of others.
If my neighbor needs me, however, I shall not avoid place or person, but will go freely, as stated above.
See, this is such a God-fearing faith because it is neither brash nor foolhardy and does not tempt God.'”