To understand emerging and reemerging diseases, you must understand the interconnectedness between human health and the environment and have a grasp on epidemiology.
Epidemiology, the study of determinates and distribution of disease in populations, is essential in protecting public health and controlling health problems. Before moving into the specifics of epidemiology, you need to understand some of the basics of human anatomy and physiology; specifically, how the immune system protects us from disease.
Your body’s first line of defense against a foreign invader is keeping the invader out. The skin is part of that defense, as it creates a barrier over most of the body. This defense continues with the mucous membranes lining your nasal pathway, and the hairs help catch particles and keep them from entering your lungs. Tears and saliva both contain lysozymes, which can break down foreign invaders. Bleeding from an open wound helps to rinse away dirt and other particles, and clotting helps keep anything from entering the body through that wound. Your body contains many different types of white blood cells that can fight off a variety of pathogens.
If an invader gets past the first line of the defense, the body’s second line of defense is the immune system. We can acquire natural immunity in two different ways: naturally acquired active immunity occurs when we are exposed to a disease-causing agent (for example, getting chicken pox as a child), and naturally acquired passive immunity occurs when antibodies are received through the placenta or breast milk. We can also attain immunity through vaccinations; this is called artificially acquired active immunity. Persons with severe immunodeficiency may be given antibody-containing serums or immunoglobins from a person or animal.
Many cells and chemicals that are part of the immune system work to destroy foreign substances as they enter the body. Macrophages circulate throughout the body and digest any foreign substances they run into. Interferons are chemicals released when a cell is attacked by a virus. These and other chemicals signal surrounding cells to shut down and prevent the virus from spreading. Lymphocytes are a type of white blood cell that produces antigens that respond to specific viruses. So, if you had chicken pox as a child, then your body will produce antibodies to protect you if the chicken pox virus enters your body again.
The state of the environment also plays a role in disease transmission. For example, the changing weather patterns associated with global warming affect disease patterns. The increased rainfall and flooding in some areas has increased the populations of a major carrier of disease—mosquitoes. The warm winters and hot dry summers in many areas are also affecting the transmission of vector-borne diseases; for example, ticks spread Lyme disease and bacteria spread cholera. There is significant evidence that outbreaks of Ebola are related to unusual patterns in the wet or dry cycle. Increases in international travel have also increased the spread of diseases worldwide. In the United States, emerging diseases such as West Nile Virus cause severe illness and sometimes death (World Health Organization, 2011). As diseases spread, or new diseases are recognized, fear of a major epidemic has caused public health agencies to prepare plans for mass epidemics or bioterrorism events.
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