In general, you are given a cannon placed at the center of the screen, and the goal of the game is to eliminate incoming viruses before they touch the cannon. You eliminate viruses by firing at them either by clicking on your mouse or touching on your screen, depending on the device you are using. If a virus touches your cannon you lose.
At the beggining of each level you are given a goal you must achieve. This can be either eliminating a certain amount of viruses or surviving for a certain amount of time. Also, in some levels you are given extra instructions like avoiding hitting white cells because this will lead to losing points or even losing the game.
On certain levels you are given 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. On some levels, bombs might also eliminate white cells. 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.
When you successfully complete a level you can continue with the next one, repeat the one you already played or exit to the main screen of the campaign mode.
The following video explains the process followed to complete an indicative level, to help you better understand the gameplay: