Try to stay low in the game to break the virus from getting into a checked pattern.
Look for large groups of blocks, use paint if needed to enlarge groups.
Focus on one color at a time if you can.
It will be easier to complete if you are lucky to get two or three butterflies the same color, so they can all be filled together.
If you have at least one block buster (hammer) booster this Level can be a lot easier...
You can stop the virus from spreading by removing the block (bottom left) that it needs to spread onto
(this can be seen in the second play through in the video example)