Oh no, the bubbles have gone sick. Hurry to get rid of all bubbles under the infected bubbles, because the plague spreads down-wise. One infected bubble will infect other bubbles underneath.You often get a chance to burst bubbles above the infection and then the infection fall into the cauldrons generating lots of points.
Infected bubbles are introduced at level 46. Special thanks to tumba25 for tipsing about the level. :)
Insight by KellyK:
Infections give priority to shots that make clusters. If you have a situation where you have two bubbles and can’t help but hit an infected bubble as well, the infection won’t take your shot away.
If you are a money player, there is a way to heal infected bubbles. You will need to buy the magic charm of antidote, it’s current price is 117 facebook coins.
Here is a video walkthrough for level 46 uploaded by skillgaming.