The movie itself doesn't say, and she doesn't seem to have any wounds. A less than HD video shows her knuckles may be bruised or scraped but maybe that's just shadows.
The original script has a slightly longer conversation, including this key part:
406: He ran right for me. Just totally wild. Violent. He grabbed me. And I dropped my phone and pulled away. And ran. So did he. He must have slipped, cause I made it inside the gate. Last I saw, he was still out there, going crazy.
406 clearly states she was touched here, unlike the movie which makes it seem like the "homeless, sick guy" only got near her and tried to bite her. Assuming that the conversation was pared down for time, it would still make sense. She got scratched or otherwise got infected by a zombie spitting or touching her (think the blood drop in the eye in 28 Days Later, which the script actually mentions in the beginning as an example of how they want to shoot the intro scene).