They can be easily spread or transmitted from person to person, cause high mortality rates and have the potential for serious public health impacts, could cause extreme concern and social disorder, and require special action to public health preparedness. Some examples of Category A agents are anthrax, smallpox, and bubonic plague. Category B agents can be moderately easy to spread. They cause moderate disease rates and low mortality rates and require specific improvements in CDC laboratory capacity and better disease monitoring. Some examples include typhoid, food safety threats such as salmonella and e-coli, and threats to water supplies. Category C agents are emerging pathogens that may be engineered for mass spread due to their availability, ease of production and spread, high mortality rate, or ability to cause severe health impact. Category C includes Nipah virus, H1N1 and HIV/AIDS (bioterrorism agent categories). Biodefense uses medical measures to protect people from bioterrorism. This includes medicines and vaccinations. It also includes medical research and preparations to defend against bioterrorist attacks. It can prove difficult to defend against an air attack with people taking trains and having so many crowded cities and places