Help with fleas

I recently moved into a new apartment like 3 weeks ago and today I noticed my cats had fleas. The only time they are outside is when we let them go on the patio but I live on second floor and our patios are screened in. Can letting them outside there cause them to get fleas? My cats never had fleas since I had them for two years but now they do. My cousin also been staying over since I moved in and she has a lot of dogs and cats. Could she possibly be bringing fleas over from her clothes I’m not sure what can cause them? Also what would be the best way to get rid of them permanently.