Answer:
Air born pathogens are minute creatures responsible for causing a variety of illness. These pathogens may be bacteria, virus or fungi and spread through a variety of mechanism. When an infected person sneeze, respiratory droplets add into air. These infectious droplets when inhaled by a healthy person he become ill.Example is flue virus.
The immune system.
The immune system acts as a defense system of the body. It fights against bacteria, viruses and any other disease causing pathogens. Some chemicals can weaken the immune system and make them vulnerable.
Water pollution is the contamination of water bodies, usually as a result of human activities. Water bodies include for example lakes, rivers, oceans, aquifers and groundwater. Water pollution results when contaminants are introduced into the natural environment. For example, releasing inadequately treated wastewater into natural water bodies can lead to degradation of aquatic ecosystems.
I believe it would be the 3rd option.
