yes
you can bring in the fleas and other parasites if you have a windows open
with a screen in it the fleas can get thru it

Believe it or not, an indoor cat can get fleas from the outside and I
unfortunately know this from experience.  My last apartment was a basement
apartment and the windows were literally ground level.  There was a stray cat
that would always come around and sit in the window on the outside and my cat,
Jack, would sit in the window on the inside and they would just stare at each
other.  After I had been living there about 6 months I noticed that Jack had
fleas (as did my ENTIRE apartment).  I could not for the life of my figure
out how this had happened and then realized that the stray cat must of had
fleas and they had managed to jump off him and onto Jack through the screen.
Needless to say, I moved shortly thereafter because I figured it would just
keep happening.  We now live happily in a second floor apartment and I no
longer put Advantage on him and he has been fine since.