Terrorists have access to graveyards, dead bodies. They can weaponize diseases from dead bodies and spread them via airport networks.

Terrorists already know how many people all over the world will refuse to wear a mask and otherwise defend against a disease.

Terrorists often believe in a "Heaven" after death and that they will be rewarded for murdering living human beings and causing ruin of Earth - so they can speed up the clock to reach this magical "Heaven".