First, get rid of the two brackets between 'prepare();' and 'Greenfoot.setSpeed(25);'. Next, assign an image to the 'InfectWorld' class (even if it a blank image). NOTE: Would it not have been easier to use a for loop as such: for (int uninf = 0; uninf < 36; uninf++) { int myX = Greenfoot.getRandomNumber(15); int myY= Greenfoot.getRandomNumber(13); UnInfected uninfected = new UnInfected(); addObject(uninfected, myX, myY); } This would add all your UnInfecteds!

Edit your scenario -- put in a description: What is does or demonstrates. What to look for that indicates state of Actors (when infected; what turned image means, etc.