flea on my baby

my baby had a black little spot on her arm for awhile. maybe an hour.. i thought it was just a little piece of dirt or something. when i looked closer IT WAS A FLEA!!!! we don't even have pets , heck we don't hang out with anyone who has pets or anything of that sort. we're not allowed to have pets here in our duplex but the neighbors have an outside cat that they feed and it comes up to our door and poops. that is the only way i can think of that they'd get in our home, our washer and dryer is out back so i'm in and out constantly ... is there anyone other way we'd get them?

i'm not a neat freak but i'm not dirty either. i'll have unfolded clean clothes lying everywhere and baby's toys but i'll never have like trash and stuff like that...

and are they harmful to her?? my poor baby. is there any at home diy to get rid of them?

edit : thank you everyone !! we have like cement floor and very little furniture since we just moved in ao looks like we'll have to do a deep cleaning very soon and try to get rid of them!