When the human immune system is low, it can cause the following major types of diseases: 1. The most common diseases are infections. If the function of the immune system is reduced, the person is susceptible to infection by bacteria, viruses, fungi and other atypical pathogens, such as mycoplasma and chlamydia, which can still lead to colds, upper respiratory tract infections, gastrointestinal system infections, urinary system infections, and digestive tract infections. 2. When the immune system is low, certain connective tissue diseases are also likely to occur, such as rheumatoid arthritis, tuberculosis, dermatomyositis, systemic lupus erythematosus, dry syndrome, and so on, 3. When a person’s immunity is extremely low, he or she is also prone to other diseases, such as infection with AIDS, infectious mononucleosis and a series of blood system diseases.