We are surrounded by bacteria, viruses, fungi and parasites. The fact that we nevertheless do not fall prey to infections is thanks to certain cellular sensor molecules such as toll-like receptors (TLR), which recognize the molecular structure of pathogens and intercede by ensuring an often completely unnoticeable elimination of the invaders. Their immune-activating abilities were only detected in 1998, a discovery which was awarded with the Nobel Prize. Now scientists are examining the complex recognition of bacteria.