How does a 3 yr old get genital warts?

my 3 yr old grandson has just been diagnosed with genital warts. we noticed he had one on his a few weeks after birth, but due to swelling from the circision i can't say with a certainty that it was there from birth and we thought at the time it was from the circision or something like a skin tag. he recently was sick with loose bowl movements and around his we thought he may have gotten little hemorrhoids or a rash from this. we took him to the doctor and it was diagnosed genital warts. how can a 3 yr old get genital warts?