Well the quickest a ‘typical’ bacteria can grow is multiplying itself about every 20mins but to do so, it has to have PERFECT conditions, i mean it needs to have the right food, the right temperature, the right amount of light, the right amount of movement, the right about of oxygen, the right pH. Bugs are fussy aren’t they?!?
I think the fastest one will probably be an E. coli, and probably one of the types that we use in labs everyday so be the basic work-horse of the microbiology world! E. coli does so much for us!!
But do you know what one of the slowest is: My bug; Mycobacterium bovis. While E. coli can generate (dublicate itself) in 10 minutes, my one needs 24hours!!!
Hope this answers your question! I’m not sure about viruses mind…bacteria has always been my thing!