I think stepping one someone else's color has no effect on this (though avoiding it regardless is quite easy)
Anyway, 3 or 4 seconds after a black obtuse, boss will target someone and immediately hit them by around 18k, less if you popped a defensive cd. That damage is unavoidable. It doesn't matter if the boss was taunted during that time, it will still switch target and nuke someone.
Two things to avoid death. First, healers must have everybody topped off after a black obtuse, so they can survive that first huge hit. Second, tank must taunt immediately after someone got that hit, failing that , boss will continue its attack and will kill that player.
The Disinfection is indeed random target, but it is not unavoidable. It all comes down to making sure people do not run over the wrong colors during Black Obtuse.
Each time a circle of your color is cleared by the correct people in Black Obtuse, you each get one stack of an effect called (Whatever your color) Convex in your debuff bar per circle, (i.e. Blue Convex, Purple Convex, etc)
This effect protects you from disinfection. If you get hit with Disinfection and you have 2 stacks from clearing both circles, you take ~4k. No biggie. If you get targeted by Disinfection and you have fewer than 2 stacks, you get hit for ~19k and will probably die.
I think Disinfection is RNG based, our 2 tanks haven't had much success figuring our taunt timing. So everyone having 2 stack of their Convex is crucial.
tl;dr --> Don't run through other people's circles in Black Obtuse, they need them to survive Disinfection. And also Disinfection doesn't always
happen after Black Obtuse