A microorganism may be a microbe that has the potential to cause sickness. To cause an infection, microbes should enter our bodies. Microbes will enter the body through the four sites listed below: Respiratory tract (mouth and nose) e.g. respiratory disease virus that causes the contagious disease. Gastrointestinal tract (mouth oral cavity) e.g. eubacteria epidemic cholera that causes cholera. Urogenital tract e.g. Escherichia that causes urinary tract infection. Breaks within the skin surface e.g. eubacteria tetani that causes tetanus.