Ehh... It seems like you tried to be biologically accurate, but viruses aren't nearly big enough to compare to red blood cells, and white blood cells are far larger than any red blood cells. Also, smallpox and HIV are completely different viruses; one would not progress into the other, as with all the diseases you seem to shapeshift into in the game.
As for the actual gameplay, there were some times when I SWEAR the red cell was smaller than me but it still hurt me. But frankly, it starts off incredibly easy, and stays really easy until the very end of the level, where red cells smaller than you are extremely rare. So you just prance about, having nearly finished, until you pick up a bonus cell that lets you eat all red cells, and then you go on a rampage and eat everything and win the level.
I can see that the progressive difficulty is some attempt to make the levels harder, and it works, but it is unclear where the boundaries of the size of both the virus cell and the red cells end, making it difficult to see which is bigger. Does the virus end at the edge of the black circle, or the edge of the white circle around it? However, the 'eat all red cells' bonus just makes it extremely hard for a while until you find it, and then extremely easy. In fact, I'd go as far as to say it's almost impossible to finish it without using sed bonus cell. I think a better bonus cell would have been one that increases your size by a fair portion without getting you any closer to your goal, so you can eat some previously larger red cells, but not ALL of them.
I also would have liked more of a punishment for being hit by white cells; at the moment, it does the same as being hit by a larger red cell (which is also somewhat unrealistic, if you were trying to be biologically accurate). I would have prefferred that if a white cell hit you, it started releasing antigens that attatch to you and slow you down, perhaps?
Also, there are some occassions where red cells appear to be the same size as you. Sometimes they can hurt you, sometimes you can hurt them. It makes me think there is a number related to the size of each cell, where the higher the number the bigger the cell, but the visible size of the cell doesn't increase every number..? (If that makes sense?) It would be cool if upon hitting a cell the same size as you (or very close to) you simply bounce off. This might be a hazard because it might push you into a larger red cell or a white cell.
One other thing that bothered me is that viruses should multiply. They wouldn't attempt to take over an entire body by themselves because that simply wouldn't work. Perhaps a sequel where there are multiple viruses, but the player only controls one at a time, and the other viruses float around, only digesting much smaller red cells, and trying their best to avoid other cells? Then, when your virus dies, you could click on another virus to start controlling that one.
It also was a bit laggy at some point, and I would have liked an option to control my virus with something other than the mouse, seeing as I have one of those laptop pad things.
The game was good though, I did enjoy it, although it was rather frustrating when I was hit by something that seemed smaller than me. Perhaps I should get an eye test soon. Very nice music, the 'Victory' screen music playing in the background as I write this is so peaceful ^-^ which, you know, seems out of place what with the being a murderous virus, but there you are. In fact, I was going to give you 3.5 stars, but this music has put it up to a 4 ^_^
This game has a lot of potential in it and could use more features to mix it up a bit (such as multiple viruses, antigens and less game-ruining bonuses), and I look forwards to Vivirion 3 in hope :)