I started by looking for passages that refer to plagues, pestilence, disease, and sickness. It wasn't long before I found the answer.
It wasn't from the Bible, though. It came in the mail. Here it is:
The U.S. constitution requires us to have a census every ten years. And this is one of those years.
But why would a census upset God so much?
Who knows? But it clearly did when King David had one.
Here's the story from Second Samuel:
And again the anger of the LORD was kindled against Israel, and he moved David against them to say, Go, number Israel and Judah.
And David's heart smote him after that he had numbered the people. And David said unto the LORD, I have sinned greatly in that I have done: and now, I beseech thee, O LORD, take away the iniquity of thy servant; for I have done very foolishly.
For when David was up in the morning, the word of the LORD came unto the prophet Gad, David's seer, saying,
Go and say unto David, Thus saith the LORD, I offer thee three things; choose thee one of them, that I may do it unto thee.
So Gad came to David, and told him, and said unto him, Shall seven years of famine come unto thee in thy land? or wilt thou flee three months before thine enemies, while they pursue thee? or that there be three days' pestilence in thy land? now advise, and see what answer I shall return to him that sent me.
So the LORD sent a pestilence upon Israel from the morning even to the time appointed: and there died of the people from Dan even to Beersheba seventy thousand men. 2 Samuel 24:1-15
And here's the account from First Chronicles:
And Satan stood up against Israel, and provoked David to number Israel.
And David said to Joab and to the rulers of the people, Go, number Israel from Beersheba even to Dan; and bring the number of them to me, that I may know it.
And God was displeased with this thing; therefore he smote Israel.
And David said unto God, I have sinned greatly, because I have done this thing: but now, I beseech thee, do away the iniquity of thy servant; for I have done very foolishly.
And the LORD spake unto Gad, David's seer, saying,
Go and tell David, saying, Thus saith the LORD, I offer thee three things: choose thee one of them, that I may do it unto thee.
So Gad came to David, and said unto him, Thus saith the LORD, Choose thee
Either three years' famine; or three months to be destroyed before thy foes, while that the sword of thine enemies overtaketh thee; or else three days the sword of the LORD, even the pestilence, in the land, and the angel of the LORD destroying throughout all the coasts of Israel.
So the LORD sent pestilence upon Israel: and there fell of Israel seventy thousand men. 1 Chronicles 21:1-14
Although the accounts differ as to who inspired King David to have the census (2 Samuel says God did it; 1 Chronicles blames Satan), both agree that the census upset God so much that he sent a pestilence that killed 70,000 men. (Neither account says how many women and children were killed. But there must have been a couple hundred thousand or so.)