How can sewage pathogens cause diseases?

Easily: A lot of bacteria for example that cause human disease are part of normal sewage, precisely because a lot of sewage comes from, well, humans. So for instance, contaminated water with the salmonella and e. Coli bacteria cause human disease when sprayed on vegetables. Luckily in the usa at least, we have mostly good sewage treatment and outbreaks are less common, but do occur.