The smallest known living cell is the mycoplasma bacterium. The cells are a type of bacteria and only grow to be 0.8 micrometers in size.
These self-replicating microorganisms lack cell walls and can be deadly to humans and animals. Made mostly of protein and lipids, the pear-shaped cells are known as surface parasites, since they can invade the respiratory system, genitals or digestive tract.
In humans, the bacteria is known to cause infections, including pneumonia, by invading the upper and lower respiratory tracts. The infection is detected through cultures, and it can be treated with specific antibiotics, such as erythromycin.