What's happens at temperatures in both the 'pasteurisation' and 'danger' zone?
What you need to understand is that making raw proteins safe for consumption is not simply a function of temperature.  It is a function of time plus temperature.  Because of the precise temperature control of sous vide, one can cook at lower temperatures IF one understands that a longer time at those temperatures renders food safe.  There is no better explanation than the work of Douglas Baldwin.  I would recommend reading that for starters.
Most food pathogens stop growing at 122F (50C).  However, clostridium perfringens can grow up to 126.1F (52.3C).  So, in general sous vide cooking is done above these temperatures.  However, there are exceptions to that.  You would likely not want to cook a steak at that temperature.