Some bacteria, viruses, parasites, and fungi that can't be seen, tasted, or smelled can cause an illness. these are known as

Pathogens are bacteria, viruses, parasites, and fungi that cannot be seen by the naked eye, be tasted by our sense of taste, nor be smelled by our noses.  These microorganisms’ causes illnesses, especially to their host, the host serves as their food, shelter and victim. Pathogens are found in wastes, such as hospital wastes or in the sewages. Pathogens are mostly parasites, which is why they need a host to survive.

weathering is the answer.