Symbionts can be acquired by an individual host organism horizontally (from the environment) or vertically (from a parent). In humans, there is evidence that infants are largely colonized by their parents' microbes, and family members living in the same household have microbiomes that are more similar than those of individuals living nearby. However, no two humans have identical microbiota, and certain factors, such as diet and antibiotics, can alter the microbiome. This indicates that humans may be colonized by .