Also biased because they are sampling people who are out and about. People who are out and about likely come into contact with more people than people who are staying at home and rarely going out.
Out, about, and willing to speak to a stranger, which I think is the biggest bias. Pretty much everyone I know goes on the occasional walk, but we all stay the fuck away from other people.
Conclusions: These results show that there is an immediate active behavioral response to infection before the expected onset of symptoms or sickness behavior.