Good god. This is why we have anti-vaccine people in the USA. People assume they know science and run with the first thing that they hear wherever.
Nitrates are found in soil naturally and are critical to a plants development. Most root vegetables require many nitrates to grow, and lo and behold, tend to be extremely high in nitrates/nitrites, followed by leafy greens. In fact, most of a humans injestion of nitrates come from the injestion of leafy greens and other root vegetables. Nitrates/Nitrates in processed foods is just celery juice, celery is LOADED with nitrate/nitrites.
Thirdly, nitrtes/nitrites actually being bad for you is extremely contested. There was evidence of a possible link in an experiment, however that experiment didn't actually enact a control group. However, otherwise, there is very little actual evidence that indicates that nitrates/nitrites are bad for you.