The biohazard warning sign is the wild symbol, it will substitute for all of the other symbols, except for the scatter symbol, to help and form winning combinations.
The scatter symbol is a brain and key to one of the bonus features.
If you land the scatter symbol on reels two, three and four simultaneously then you will win five free spins. During the spins, three special zombie wild symbols will roam the reels and replace any symbols they land on with a wild symbol. Furthermore, the scatters symbol is replaced by a special feature wild symbol that will substitute for all of the other symbols and award a 3x multiplier when it helps to form a win if a zombie wild lands on it. If a zombie wild lands on one of the character symbols then you will receive a free respin of the reels during which the zombie wild will replace the character symbol.
Scatter Brains 2 offers a top payout of 6 times your total bet for landing five of the red character symbols along a payline. The scatter symbol will payout 5 times your total bet when three land on the reels. The orange character pays out up to 2.4 times the total bet, the blue character up to 2 times the total bet, the green character up to 1.5 times the total bet, and the purple character up to 1.2 times the total bet. The four playing card symbols can all payout up to 0.5 times the total bet.