How do you launch a spike?

The other answer is (edit: was) incorrect. Space will split your cells into half, and when these cells go over the virus they will just split more, not feed the virus.

To launch a virus, you should hover your mouse in the direction of the virus (careful not to run into it) and spam W. This will spew out tiny food parts from your cell which will feed the virus.

When the virus gets big enough from being fed, it will split into 2 smaller viruses, shooting out in the opposite direction to where the last food part got absorbed.

Keeping away from viruses is the best way to avoid this. You can also counter-feed the virus in the opposite direction, hoping feed it last before it splits and thus firing the virus towards the person attempting to split you.