There are two possible solutions, both of which have already been given. "All" can refer to one; there is nothing about the word that denotes multiples. It just means completeness. The bit about the "same day" carries a connotation of multiple monks, but, due to the vagaries of English usage, could refer to a solo monk as well. Word puzzles do that sort of misdirection. So a solo monk could be all the infected monks and would kill himself upon not seeing any other dots. Since he was "all" the infected monks, it would logically follow that he killed himself on just one day, which would be the same day as all other infected monks (in this case, zero) killed themselves.

Or two monks with dots would each know they were infected on day 3, as explained by Nigel.