Less than 1/3 of the cells in your body are human.

Bacteria in the human body, also known as the microbiome, perform many vital functions that help keep us healthy:
Digestion: Bacteria help break down food and make nutrients available to the body. For example, bacteria in the large intestine continue digesting food particles that weren't fully digested by the stomach and small intestine.
Vitamin production: Bacteria produce certain vitamins that the body needs.
Immune system regulation: Bacteria help regulate the immune system.
Protection against disease: Bacteria protect the body from disease-causing bacteria by destroying disease-causing cells and protecting colonized surfaces from pathogens.
Reproductive health: Bacteria help maintain reproductive health.
Body odor: Some bacteria give humans their distinctive body odor, especially in the armpits.