My team had such a thing in the early 2000s, with either G/S or R/S/E, although it didn't change that much. Such things can provide a small incentive to keep on playing, although it could perhaps be better formulated. You figure that they kind of improved on this aspect with the hidden island, etc., in R/S/E. In general, it seems to have a bit in common with shiny Pokémon and wandering legendaries, things which have a chance of showing up and hence keep things going over an indefinite time, rather than the game being merely linear. Likewise, the fairly free-roaming Kanto area was also vaguely in this direction, although of course part of the same progression. Pokérus in particular seems a bit niche, though, and wouldn't necessarily motivate players as much as the other two, but it does include a certain, interesting element of variation, as well as perhaps raising the possibility that Marill is merely a Pikachu with midichlorians. The more recent games tend to be generally more direct about such things, though, and as such don't complement such approaches as much.
I have just traded a pokemon and I got a Talonflame lvl 100 with pokerus, and I have never heard of it, so I saw it's a poke virus and now I want to know: Is that a good thing?
That was not clear for me, because some people usually offer to trade a pokemon with pokerus but what does it really change?
It's generally seen as a 'good' thing, yes. It improves some things such as levelling up and related hijinks, although it's not that major. This might seem strange, given the label, but you suppose they didn't want to throw debilitating things at the players in such ways, and as such they're vaguely similar to shiny Pokémon.