Interesting game, I liked it. The concept worked for me.
I think if the bullets had been larger or slightly slower it would have been easier to track. I re read the tutorial and I got how I was infecting others (first couple of attempts I did not get how), a visual reminder would be a nice touch. I'm curious why you chose to not refresh the life of the enemy you infected, so you would start each enemy with full health. I get why the health decreased because I shot it, but personally I was confused when I respawned into an enemy with 1 or 2 health, one time dying right away to a bullet.