I selected a piece of text in Word and started typing new text to replace the
selected text and then the new text gets red and with a line trough it. I
used no red text in the whole document and definitely not any text with a
line through it. I can't figure out how my new text got formatted in that way.

Can it be a virus on the file? I scanned it and there was no sign of virus.

Is it a function in Word that performes this itself?

Does anyone know?