You tell everyone. Head lice isnt about 'dirt' - its simply an 'infection' like athletes foot or a cold.
Everyone in the flat needs to use a good head lice lotion (from a chemist not a supermarket) and you need to wash towels, bed sheets etc. Also check cushions, and anywhere else you stick your head. Unless you are sharing with some very silly people they will all see the sense in you all clearing this problem together. There is no need to apportion blame - just 'Fred has got head lice, I think I have too so we all need to use the head lice stuff to be certain.'