Any virus or bacillus can be very dangerous and unpleasant in your blood circulation until you play as the alien. The main job is to infect the blood cells, reproduce yourself and survive. As always in this type of game there is some evolution. Here, it's because the leukocytes and antibiotics making your bad day. The gameplay is quite good but you will get through a lot of clicking. The music can annoy and it cannot be turned off.