Flies spread diseases because of their breeding and feeding habits. Bacteria from where the fly usually feed on would get stuck on their mouth parts and footpads eventually spreading onto places they land on. Imagine if it’s exposed food that you are about to eat. 

mosquitoes are the only species known to carry malaria. They also transmit filariasis (also called elephantiasis) and encephalitis.

From the transmission of bubonic plague to typhus and hantavirus, rat infestations can prove harmful to human health. Rats also are a potential source of allergens. Their droppings, dander and shed hair can cause people to sneeze and experience other allergic reactions.

Bed bugs can be an annoyance because their presence may cause itching and loss of sleep. Sometimes the itching can lead to excessive scratching that can sometimes increase the chance of a secondary skin infection.

cockroaches are known to transmit the Salmonella [sp.] bacteria which can cause salmonellosis, a disease in humans with symptoms similar to food poisoning. It is believed that cockroaches accumulate the bacteria by feeding on contaminated food.

ants can transport several different food-borne illnesses, including some as serious as salmonella.

Innumerable microorganisms live in a spider's web, which is harmful to our health. Along with this, they also become the reason for discord in the house and this destroys happiness and prosperity.