People kill cockroaches because they can pose a potentially harmful threat to a person's health and well-being. Some of the main ways cockroaches can inflict damage on people include infesting food, passing on illnesses and causing allergies.
Cockroaches eat almost anything and live in generally unhygienic environments with fecal matter, dead plants and animals, and bacteria-riddled garbage. Cockroaches also flock to and eat human food, bringing in particles of what they have been eating and living in, and contaminating food. Cockroaches can also defecate or lay eggs in food, posing a health risk to any humans who eat it.
Most species of cockroaches leave traces of their saliva and digestive fluids in the foods they eat. Bacteria live in these fluids that are foreign to the human system and have been documented to cause serious health issues and illnesses such as sepsis and urinary tract infections.
Hundreds of allergens also live within cockroach saliva and digestive fluids. People who are susceptible to these allergens can suffer side effects such as skin rashes and congestion. The allergens released by cockroaches, in addition to the materials they track into a dwelling, can also cause serious problems for individuals with asthma or allergies.