Pulmonary fibrosis foci are fibrotic lesions left over from natural healing after a lung infection, and can also be described as fibrotic changes in the lesion; normally the lung tissue is loose, soft and porous; however, with fibrosis, the lung tissue hardens and becomes more solid; when the fibrotic lung tissue is viewed under a microscope, many thin strands of translucent and red material are found scattered or collected in the normal alveolar tissue of the lung.