Pili are hair-like structures in bacterial walls that allow bacterial cells to adhere to other surfaces throughout their environment. Pili are also known as fimbriae.
Medically, pili are virulence factors for pathogenic bacteria. A virulence factor is any property of a bacterial cell that allows the bacterium to infect another organism. Pili are considered virulence factors because they allow bacterial cells to adhere to tissues and can help the bacterial cells resist attack from immune cells in the human body. In turn, the bacterial cells can colonize the tissue and spread throughout the body, causing the organism to become sick.