There are many categories of agents that cause communicable diseases (I'll give an example of a few:)
Viruses, such as Rhinovirus which causes the common cold, or the Herpes Simplex Virus Type-1 which causes the cold sores that some people get on their lips.
Bacteria, such as Staphylococcus aureus which can cause serious infections of the skin or blood; Neisseria meningitis which causes meningitis, an infection of the brain, or Mycobacterium tuberculosis which causes tuberculosis, an infection of the lungs.
Fungi ("mushrooms") such as Candida albicans which causes thrush in the mouth or vaginitis in women.
Protozoa, such as Entamoeba histolytica which causes some forms of diarrhea, etc, etc.