Microplastic

 were microparticles that were obtained from plastics that couldn't be fully broken down. The microfiltration system in the water could filter out microplastic.

By early 2020s, humans have flooded planet Earth with plastics so much, that microplastic was present in the air, in the food, in the water. According to the Thirteenth Doctor, even humans were full of them, since they were ingesting microparticles whether they new it or not.

Praxeus was homing in on microplastics. It led to the birds and some humans being infected with it. (TV: Praxeus)