Thanks! viruses can be quite tricky and have to admit that I may have made them a bit too hard to beat.
you can shoot bacteria and every shot makes bacteria bigger. When bacteria gets big enough, it explodes. But when you shoot bacteria, it directs it's movement towards you. When you move, you are not at the "target point" anymore. You can use this mechanic in your advantage also.
There is a bit minus gravity in the movement of bacteria, so it curves up a little and makes running away easier.
I hope this helps you to beat them.