Infiltration is a result of negative pressures in the building or positive pressure on the outside.
Whether in a commercial kitchen or in restrooms, exhaust fans draw a significant amount of conditioned air out of a building. Since this is air you have just paid to cool and dehumidify, you want to prevent releasing any more air than is necessary. The real culprit of infiltration is the outside air or attic air that is drawn into the conditioned areas due to the negative pressure the exhaust fans exert in the building. Ventilation requirements and standards are usually based on occupancy levels and internally generated air pollutants. For a given type of use, a specific amount of fresh air intake is recommended to maintain healthy conditions.