Don't be. With good maintenance, chances are slim that a new fish will kill your stock. If you are worried, have a quarantine tank ready for any stock you buy.
Well I actually only have one betta
and I've had him for over a year and he hasn't gotten me sick...But now I'm worried about any bettas I get in the future, like after my current one. Not only am I afraid of myself getting sick, but now I am actually extremely depressed to learn that pretty much every pet fish I will ever have will have this illness :..(
2.5G - 1 female betta: "Miss Cleo"
5.5G - 1 betta: Half-moon King "Cobalt"
10G (with 4G of water) 1 female betta "Grace"
29G - 1 gold marble angelfish "Bartholomew"
1 Male fancy guppy "Guplet"