Gawd, there is white cells everywhere
Well, the game need the white cells and the antibiotic clouds, or it would be too easy, right.
But here, right after the campaign (not very difficult, but hey, it's a tutorial, no ?), with the endless mode, you just have tons and tons of those white and blue clouds at the very beginning of the level, and if you're not lucky enough to have your first germ immunised, it's already dead before you can even see a red cell. And at this point of the game, you don't have enough points to be specialized in immunity, or reproduction, or anything; you just don't have the weapons to face THIS HUGE ASSAULT of clouds covering the whole screen and occulting the few red cells. So you try to survive, with one single germ slaloming between white cells, stalling, and not earning points (points needed to evolve). Then, impossible to progress.
This kind of level would be great later. The main problem of the game is not that it is hard, but that it is really hard way too early.
That, plus some lag, and a tiny screen.
I would add something I find confusing: why are the germs even leaving the screen? I mean, we are in a blood vessel, right? So the germs should be hitting the vessel's walls at least on the sides of the screen, instead of disapearing in the infinite void of... of what? Muscles? It just doesn't make sense.
In the end, not a bad idea, not even a bad game, but the points above... So 5, because it's not that bad, but only 5, because it could be really better.