Not that hard, but gets tedious
I got up to stage 37 before stopping, so it's addictive enough to keep me playing that long, but by stage 30 nearly the entire screen is covered in antibiotics, and those mysterious effectively invincible black blood cells are block off a good chunk too.
As a result of the bug/feature that dieing viri can still infect red blood cells, you can continue to surivive as required for endless mode, but regardless of your stats it basicly ends up being a repetitive infect, grab a single virus and quickly infect another one before it dies (usually nearly immediately due to the black and white cells everywhere, not to mention the antibiotics), repeat, with essentially no potential to amass any singificant amount of viri. The huge amounts of lag at that stage even with only a couple viri on screen doesn't help things either.
While I appreciate that since theres a high score feature so being able to go on indefinitely shouldn't be easy, right now you pretty much can, but it's not fun. If one could evolve to a point where you were basicly an unstoppable super virus that can pass stages with little to no interaction, then I think that would be a reasonable end game state. As it is though, black blood cells pretty much take over, and don't seem to be defeatable even at 10 damage and lots of viri.
Also, the latency upgrade doesn't seem terribly useful, particularly once I got to the endless levels where it was more of a hinderence, since the only 'safe' place for your virus to be is busy infecting a cell, at any given time outside of that it's probably going to be in the process of dieing.
I should also point out that the black death medal seems to be broken, as I earned a total of 76730 EP but didn't get it. If you need 70k total *unspent* EP, then that's just unreasonable, since that'd take hours of basicly babysitting a single virus at a time as I outlined above.
So basicly, I'd recommend taming those black cells somewhat, and possibly making the higher levels of damage more effective so our viri can actually stand a chance to build up, and potentially make the Pandemic medal more feasible. On the early campaign missions you generally complete the objectives before you can get to 65, and even pouring all yourpoints into reproduction/immunity it doesn't seem possible without a lot of luck, and it's pretty much impossible on the endless levels to have more than a dozen alive at once, and even that requires a lot of luck.