Changes in the environment occasionally increase the incidence of infectious diseases. Regrowth of forests in the eastern United States with cessation of farming has led to massive increases in deer and mice, which carry the ticks that transmit Lyme disease, babesiosis, and ehrlichiosis. Global warming also has had an impact on the spread of infections. For example, the mosquitoes that carry Dengue fever and Zika viruses, which used to be confined to the U.S.-Mexican border, are now found in more than half of the states. Chikungunya virus transmitted by mosquitoes, which causes fever and joint pain that can be severe for some, was first seen in the Americas in 2013, and transmission of Dengue and Zika viruses has recently been reported in Florida.
Infectious diseases that are common in one geographic area may be introduced into a new area due to increased travel or movement of infected animals, invertebrates, or birds. For example, West Nile virus has been common in Europe, Asia, and Africa for years but was first described in the United States in 1999, possibly transported by an infected mosquito or bird. Highly pathogenic H5 influenza viruses, which have led to death of some patients in Asia, have spread throughout the world during the last 2 decades in bird populations, due to their natural migration and transport of domestic birds.
Pathogens adapt rapidly to selective pressures exerted by widespread use of antibiotics. Antibiotic resistance has developed and is now common in Mycobacterium tuberculosis, Neisseria gonorrhoeae, Klebsiella pneumoniae, and S. aureus. Similarly, development of drug-resistant parasites has dramatically increased the morbidity and mortality associated with Plasmodium falciparum infection in Asia, Africa, and Latin America.
Sadly, the anthrax attacks in the United States in 2001 transformed the theoretical threat of bioterrorism into reality. The Centers for Disease Control and Prevention (CDC) has evaluated the danger microorganisms pose as weapons on the basis of the efficiency with which disease can be transmitted, how difficult the microorganisms are to produce and distribute, what can be done to defend against them, and the extent to which they are likely to
Table 9.5 Potential Agents of Bioterrorism
Anthrax:Bacillus anthracis
Botulism:Clostridium botulinum toxin
Plague:Yersinia pestis
Smallpox:Variola major virus
Tularemia:Francisella tularensis
Viralhemorrhagicfevers:Ebola,Marburg,Lassa,others
Brucellosis:Brucella spp.
Foodsafetythreats:Salmonella spp.,Escherichia coli O157:H7,Shigella, others
Glanders:Burkholderia mallei
Melioidosis:Burkholderia pseudomallei
Psittacosis:Chlamydia psittaci
Qfever:Coxiella burnetii
Typhusfever:Rickettsia prowazekii
Mosquito-borneencephalitisviruses:Venezuelanequineencephalitis,Easternequineencephalitis,Westernequineencephalitis,others
Watersafetythreats:Vibrio cholerae, Cryptosporidium parvum, others
Emerginginfectiousdiseasethreats:Nipahvirus,hantavirus,others http:// emergency.cdc.gov alarm the public and produce widespread fear. Based on these criteria, the CDC has ranked bioweapons into three categories, designated A, B, and C (
Table 9.5