Fun at first, but soon gets repetitive!
It's a fun game, and I played for a few levels. However, the upgrades are very, very small, the pace is quite slow, and there's no variation aside from this incremental evolution and increasing amounts of blood. I kept thinking there would be something new lying around the corner, but there never was, which left me feeling a little cheated.
I'll admit that I'm not really someone who enjoys playing long, slow, repetitive games to get a l33t high score (e.g. Tetris, Columns); I prefer games which are fairly brief, have some sort of on-going story or at least some variation as levels advance, and adds new elements as you continue playing. You've got a fair number of tens, so I'm probably not your intended audience, but I see that a few other people left similar comments. So if you wanted to add some awesome for this ornery bunch, consider:
* more obvious/interesting upgrades, with more visible and elaborate changes in appearance;
* an obvious progression as levels increase (e.g. moving through different areas of the hosts body, perhaps some sort of running narrative); and
* something other than conventional blood cells in later levels.
Any one of these would have seriously improved the game, from my perspective!
To recap: it's a good game that gets dull very quickly from my possibly biased perspective, and I'd love to see what would happen if you tried a few of those suggestions in a "Parasite X-II". :)