Terrorism refers to the use or threat of force or violence against people or property. A bioterrorist attack releases viruses, bacteria, or other germs to cause illness or death. These biological agents are typically found in nature. But they can sometimes be made more harmful by increasing their ability to cause or spread disease, or to resist medical treatment. Biological agents spread through the air, water or in food. They can be very hard to detect, and they don't cause illness for several hours or days. Some agents can also be spread from person to person. Scientists worry that anthrax , botulism , hemorrhagic fever viruses such as Ebola, plague , or smallpox could be used as biological agents. Biodefense involves medical measures to protect people against biological agents. This means medicines and vaccinations. It also means medical research and preparations to defend against bioterrorist attacks.