How do pathogenic microbes cause disease? Though each organism has a specific, unique signature of creating disease in human hosts, we can identify common stages in the development of an infectious disease.
Symptoms of disease are usually caused by structural or functional changes in molecules in the cells that make up our tissues. these changes result from physical or metabolic injury to a cell, which can be due to a pathogenic organism.
These are the early stages of the inflammatory process. Local tissue damage or pathogens infecting our tissue can set off this response. The roman doctor Aulus Cornelius Celsus who lived around the beginning of the Christian era, described the 4 classical symptoms of inflammation: redness, swelling, pain and increased temperature of inflamed tissue. These symptoms can be explained by an increased blood flow to the inflamed tissue; causing redness and increased temperature, as blood is warmer than our skin normally is. As the increased blood flow increases the pressure in the small capillaries, some of the plasma may leak out of the blood vessels, causing swelling of the tissue and pain through pressure on the nerve endings.
These responses occur within minutes to hours after the initial damage. The increased blood flow allows cells to travel to the place of damage to start making repairs.
Cells that migrate to the infected tissue have different functions. The first cells to arrive can recognize, kill and remove foreign microbes. Others can kill infected cells. The destruction of cells by our own immune system is one of the reasons that we suffer symptoms of a disease. Some organs, such as brain or lungs, are more vulnerable to the effectes of infection than others. For example, if our lung tissue is infected and the defense system causes fluid leakage, then an organ such as the lungs suffer from decreased ability to exchange oxygen.
Sometimes the response of the immune system is excessively strong, destroying also healthy surrounding tissue, which also causes more severe symptoms. This is why the immune response depends on careful regulation.
In the later stages of vascular response to infected tissue cells arrive that facilitate clean up and repair of the damage. Repairs are important, because destroyed tissue is more susceptible to pathogens.