The word plague is mentioned throughout the OT, being seen as divine chastisement involving various pestilences and curses (Lev 26:21; Num 11:33, 14:37; Deut 28:21).
The most notable are the successive afflictions by which God humbled the pride of Egypt, made clear His own supremacy, and prepared the way for the exodus of the Israelites. These 10 plagues were: 1/. The river Nile was turned into blood and its fish were destroyed (Ex 7:14-25); 2/. Frogs appeared everywhere (Ex 8:1-15); 3/. Lice appeared everywhere (Ex 8:16-19); 4/. The plague of flies (Ex 8:20-32); 5/. The livestock in the fields died (Ex 9:1-7); 6/. Boils broke out on the Egyptian’s bodies (Ex 9:8-12); 7/. Hail destroyed the unprotected slaves and animals in the fields (Ex 9:13-33); 8/. Locusts continued the work of desolation caused by the hail (Ex 10:3-15); 9/. Three days of darkness (Ex 10:21-26); 10/. The sudden death of all the firstborn males of men and animals (Ex 11:4,5, 12:29,30). The Egyptian magicians were able to imitate the first two plagues but no others.
None of these disasters or phenomenal happenings were experienced by the Israelites. Ultimately Pharaoh, who had been blocking the Israelites release from Egypt, relented and let the people go. This mass migration of the captive Israelites from Egypt was the exodus. God said Pharaoh had been raised up for the purpose of showing to mankind God’s awesome power (Ex 9:16).
There are several references to plagues in the Book of Revelation: the three plagues of fire (Rev 9:18,20); seven last plagues (Rev 15:1,6,8); God’s people escaping the plagues experienced by Babylon (Rev 18:4,8); and a severe warning not to distort the message in the face of similar plagues (Rev 22:18). “If you make the Lord God your dwelling place, He will keep you safe from all the harmful plagues” (Ps 91:1,3,9,10).
Down through the ages widespread diseases and epidemics have been called plagues. Jesus healed many people of such bacterial diseases (Mk 3:10; Lk 7:21).