This page Bleach claims
Bleach is more effective at killing germs when diluted than when used straight out of the bottle.
this other page (Spanish) Alcohol and Bleach are more effective against Coronavirus diluted claims the same
Disinfectants like bleach and alcohol alter the virus structure and prevent it from infecting us. Their effects will depend in the right concentration
But this other page, Does liquid bleach has to be diluted to be effective as a virus killer?
So, all things being equal, undiluted bleach is more effective at killing stubborn bacteria.
I know, he says bacteria and not virus but,
The data of viruses is a little more straight forward � viruses are simply not cut out to defend against the bleach onslaught! A group of scientists in New York studied the effects of very dilute concentrations of bleach on the respiratory syncytial virus (RSV). A tenfold dilution of bleach, which subsequently mixed with an equal volume of RSV-containing medium (so in fact a twentyfold dilution) eradicated all of the virus. A 100-fold dilution of bleach killed 100% of the virus half of the time, and decreased the number of live viral particles by greater than three logs in the other half of the tests. This was all after five minutes of treatment.
So at least for viruses, you can probably dilute the bleach tenfold without worrying too much about decreased antimicrobial activity.
Personally, I don't find any logic why bleach would be more effective diluted, the opposite makes more sense to me. Is there any explanation for this to be so? Or it's just a wrong idea?