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Plague
Plague, has appeared in three major epidemics - in the 6th, 14th, and 19th centuries. They are known as the Justinian Plague, the Black Death and the Third Pandemic, respectively.
By the middle of the 6th century, the Emperor Justinian had spread his empire throughout the Mediterranean and Europe. He had hoped this work would bring forth a long-lived dynasty. His dreams were shattered when in AD 540, flea infested mice brought by ships from Egypt reached the harbor town of Pelusium . From there, the cruel disease spread to Alexandria and Constantinople, Justinian's capital, and continued throughout the empire. After the plague's devastation ended in AD 590, it had killed as many as 100 million people -- half the population of Europe.
The plague of the 14th century is believed to have first surfaced in China in the early 1340's. From there it spread along trade routes to India, Egypt, and Asia Minor. Word of this horrible disease reached Europe before the disease arrived in Italy in 1347. 
It took a staggering toll in the Italian peninsula and then began a sweep through Europe. In 1348 it reached England and Russia in 1351. The people of the middle ages suffered tremendously with death tolls between 20% and 40% in their cities and communities. In some areas, as many as 60% and higher died, often forcing people to abandon their towns and villages. Adding to the misery of these kinds of losses was the mystery as to the cause of the almost certain death in those who contracted Plague. Common beliefs about Plague and its origin varied. Some said it crept out of cracks in the earth left by earthquakes. Others said it was a judgment of God. And some blamed other groups of people for conspiring and poisoning their wells. One third of Europe is believed to have perished during this time.
The Third Pandemic began in China in 1855, spreading the bubonic plague to all inhabited continents, and ultimately killing more than 12 million people in India and China alone. According to the World Health Organization, the pandemic was considered active until 1959, when worldwide casualties dropped to 200 per year.
In 1894, bacteriologists Alexandre Yersin and Shibasaburo Kitasato independently isolated the responsible bacterium causing plague. Yersin further determined that rodents were the likely common mode of transmission. 
Today Plague is well understood. Modern antibiotics and vaccines are effective treatments for plague. Outbreaks in people still occur in rural and urban settings globally as well as in the United States. The most common form of plague is bubonic plague. Normal symptoms include swollen and very tender lymph glands, fever, chills, severe headache, and extreme exhaustion. Pneumonic plague occurs when the lungs become infected, allowing for airborne contaminants to infect others. Septicemic plague occurs when plague bacteria multiply in the blood.
Plague is a bacterial infection caused by Yersinia pestis that harbors in small animals and is transmitted through the bites of infected fleas. Plague presence can seem to disappear when it circulates in the animal to flea to animal cycle. During these periods where few or no human cases surface in a localized area (often lasting for years) assumptions about plague eradication should not be made as plague may suddenly re-appear. Most humans contract plague through the bite of an infected flea. Occasionally it is contracted when an infected human with advanced stages of plague coughs, expelling tiny droplets containing the bacteria into the air. As it is inhaled by another susceptible person, the bacteria establishes itself in its new environment.
Attempting to eliminate rodents and their fleas in plague infected areas is impractical. Keeping rodents out of indoor areas inhabited by humans and managing them around building and recreational areas is much more feasible, and will reduce risk for exposure to the disease. 
To accomplish this it is important to make any available food sources rodent-proof. Dog food, garbage and stored foods must be kept in containers to eliminate or reduce your chances of contact with rodents or their fleas. Snap traps and poisons may be effective at killing rats and mice, but fleas quickly abandon a dead host in search of another. By eliminating the rodent you could become the next host of a plague infected flea. Treatment with chemicals to kill rodent fleas is effective but should be left to professionals.
In the United States, the last urban plague epidemic occurred in Los Angeles in 1924-25. Since then, human plague in the United States has occurred as mostly scattered cases in rural areas (an average of 10 to 15 persons each year). Globally, the World Health Organization reports 1,000 to 3,000 cases of plague every year.
Educating yourself about fleas and their behavior, before visiting areas where plague is known to exist, is a good idea. Applying repellant to you and your family, putting flea and tick collars on pets, staying on trails
and out of grassy areas and taping pants to your shoes or boots all help reduce the risk of flea bites and potential plague transmission. Where plague is historically known to surface in California, a vector control district or another government agency will have surveillance programs in place for early detection. They have authority to close recreational areas that people visit to prevent plague transmission to humans. Flea and rodent control measures will be implemented to slow or stop the bacterial amplification cycle going on between rodents and fleas. Plague is rare in the U.S. and continuing vector management will help moderate the possibilities of acquiring this disease.
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