Microorganisms causing disease are known as pathogens. Pathogens can be BACTERIA, viruses, fungi, or parasites. The two most common types of pathogens are BACTERIA and viruses. BACTERIA are single-celled organisms that can cause a variety of diseases, such as pneumonia, tuberculosis, and food poisoning. Viruses are even smaller than BACTERIA and can cause illnesses such as the flu, measles, and HIV/AIDS. Both BACTERIA and viruses can be spread through contact with an infected person or animal, contaminated food or water, or contact with contaminated surfaces.