The document discusses the work of the National Center for Emerging and Zoonotic Infectious Diseases (NCEZID) which studies highly hazardous bacteria and viruses like anthrax and Ebola. NCEZID conducts long-term studies on Ebola survivors and investigates unexplained illnesses which has led to the discovery of new diseases. Infectious agents can be bacteria, viruses, fungi, protozoans or helminths. AIDS occurs when the immune system is damaged by HIV, causing life-threatening infections and illnesses with symptoms like rapid weight loss, fever, tiredness and diarrhea.