Alex, you need to tell the readers more about your growing conditions, especially where you are in the world. Inside or outside culture?and so on.

My best guess is Spider Mites, and I call them the silent killer because they are hard to detect, but they will cause that kind of a damage.

Insecticidal soap with a little alcohol sprayed on once a week for three weeks may help. You will probably lose this leaf, but may save the others. Good luck!

Looks like virus for me.