Poison makes different Pokemon faint?

[nicsonfire4]

I was on my to Violet City,when I come across a trainer. He uses poison sting, and my Chikorita is poisoned. He then attacks my Chikorita, which faints . I swap to my second and only other pokemon, Hoppip, just caught and at 2 hp. I then says, "Chikorita was affected by poison", and my Hoppip faints due to poison on my Chikorita ? Please fix.