Yellow Fever is the condition humans experience when infected with the Yellow fever virus, a flavivirus. A "flavivirus" comes from mosquitoes and ticks. In the case of yellow fever, the disease is contracted by humans through the bite of infected Aedes aegypti mosquitoes. Yellow fever is most commonly found in tropical areas of Africa and South America. Consequently, countries such as the US and Canada require their residents to obtain the Yellow fever vaccine prior to travel to specific countries. In addition, there are countries that will not permit entry without an "International Certificate of Vaccination or Prophylaxis". Usually the immunization must have been received within the last ten years.
Yellow fever is characterized by three stages of symptoms. In the initial stages of the disease, patients may experience severe headaches, back pain, muscle aches, nausea, fatigue, loss of strength, fever and chills. Having the virus in the blood is referred to as "viremic" and may exist three to six days before actual symptoms present themselves. For many people, they do not progress past the first stage.
The next stage is considered the toxic phase. At this point, the original fever may have withdrawn but a new higher fever exists. Symptoms may include pain in the stomach, back pain, headache, nausea, vomiting, and fatigue. More severe symptoms begin such as black vomit, nose bleeds, gum bleeding, bruising, proteinuria (urine contains too much protein), and advanced jaundice. It is because of the jaundice that yellow fever received its name.
The last stage of the disease is particularly dangerous, as patients may experience hypotension (low blood pressure), shock, metabolic acidosis (excessive acidity of the blood), acute tubular necrosis (death of body tissue), myocardial dysfunction, cardiac arrhythmia, seizures, coma, kidney failure, and other bacterial infections. For those that do recover, they usually develop an immunity to yellow fever, and may be quite weak and tired for many months after the disease has run its course.
Indeed, yellow fever research was imperative, as the disease is so dangerous to humans. Further, in populations that do not receive vaccination, the fatality rate is quite high, estimated anywhere from fifteen to fifty percent. This is most likely due to several factors. There is no real medication to treat the yellow fever per se. The symptoms can be treated, but not the virus itself. Further, the inhabitants of areas where epidemics are a problem tend not to have the resources to take preventative measures. For example, installing screens on open windows, sleeping under mosquito nets, wearing insect repellent, and unfortunately other factors over which individuals have no control. When governments allow deforestation, for example, people are exposed to the mosquitoes.
Additionally, yellow fever research determined that the virus is not contagious from human to human. A human must be bit by an infected Aedes or Haemogogus mosquito. But epidemics can start in areas where the specific species of mosquitoes breed. If one person is infected and the mosquito becomes infected, it then transmits the disease from human to human. The other problem is that the secondary infections that the person may contract might be contagious, thus, knowledge was required to enable the medical community to properly treat all symptoms.
Obviously, yellow fever research was crucial in order to develop a vaccine to prevent the widespread epidemics that have previously occurred. Inoculation is the only real prevention of yellow fever. For most people, it becomes effective within one week and research has found that it may last as long as thirty-five years. In areas where outbreaks can be expected, at least sixty to eighty percent of the population must be vaccinated. Sadly, in the most vulnerable areas of Africa, this has not happened.
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