Your goal in survival mode is to survive as long as possible. In order to survive you have to eliminate incoming viruses by firing at them. You fire at a virus either by clicking or touching towards it, depending on the device you are using. When a virus is not eliminated before coming too close to your cannon at the center of the screen, the game is over and you receive your score, which represents the number of viruses you eliminated. If your score is high enough to be included in the top 10 scores ever achieved in the game, you are given the option to save it and include your name in the pantheon of the game! Local (device) highscores are automatically saved (deleting cookies might lead to loosing local highscore).
To help you survive longer in the game, you are given a couple of extra tools.
The first one is "Rapid Firing". If you hold your click (or touch) and move around the screen, your cannon will fire rapidly until the "Rapid Firing Capacity" appearing on the top-right corner of the screen empties. This capacity refills as you go on, when firing regularly.
The second one is "Bombs". At the lower-left corner of the screen you will see a button named "Bomb" and 3 bombs. When you click the button (or when you hit "Space" in a keyboard) all viruses appearing on the screen at the given moment are automatically eliminated. You can use this feature only 3 times since it does not get refilled during gameplay.
Moreover, you can pause your game if needed either by clicking the relative button on the right-lower corner of the screen or by pressing "P" on a keyboard.
There are 3 types of viruses appearing in the game. The small green viruses, the medium yellow viruses, and the big red viruses. The red viruses after 1 hit become yellow, while yellow viruses after 1 hit become green. Green viruses are eliminated after 1 hit. This means you need 1 hit to eliminate a green virus, 2 hits to eliminate an initially yellow virus, and 3 hits to eliminate an initially red virus.
You can watch the following video to better understand the gameplay: