Seriously I was like this
for a moment till she explained.
It was actually really clever. They all had to walk round the room, each holding a test tube. Test tube contained some white fluid . Two test tubes had been spiked with another invisible substance (representing the STI). Every time the music stopped they had to share fluids with somebody . Some people had stoppered test tubes so they couldn't share fluids. At the end (after three 'exchanges') they used some indicator something and revealed that the 2 infected tubes had infected 12 others. In total flap of misapplied anxiety I asked dd1 'which group were you in'
'Oh, I wasn't infected' she replied
'Jolly well done darling' sayeth I 'Is that because you had a stopper in'?
They do NOT tell you about this sort of thing at antenatal classes.........