Albert Barnes Commentary Numbers 26:1

"And it came to pass after the plague, that Jehovah spake unto Moses and unto Eleazar the son of Aaron the priest, saying," — Numbers 26:1 (ASV)

After the plague - These words serve to show approximately the date when the census was taken and suggest the reason for the great decrease in numbers that was found in certain tribes. Compare Deuteronomy 4:3 and the note on Numbers 26:5 in this chapter.