Dear Student,
Water is a breeding platform for bacteria, insects, parasites, fungi etc. It's much easier to breed in stagnant water than in running water. Mosquitoes are the host of various diseases such as malaria,dengue, yellow fever, etc. Moreover, stagnant water can also promote the growth of mould and mildew, which can cause respiratory diseases.
Stagnant water can also be harmful for drinking purpose because it acts as a good incubator for the growth of parasites, insects and bacteria. In certain areas of deserts, stagnant water is often contaminated with animal and human faeces. So, there should be not stagnant water near your home.
Regards