What happens in yellow fever, exactly?

What is it?

Yellow fever is a viral disease. In most cases it leads to relatively mild symptoms including fever, chills, nausea, and muscle pain, and these resolve within a few days. However, in about 15% of cases, the illness proceeds to a toxic phase. This presents itself as more fever, a yellowish skin and abdominal pain. The toxic phase can kill as many as half of those who suffer from it.

Yellow fever is mostly seen in the tropical parts of Africa, and it is also quite common in South America. Every year the disease leads to about 200 000 infections and 30 000 deaths.

What causes it?

Yellow fever is caused by the yellow fever virus. It is transmitted through a mosquito bite.

What happens during it?

Viruses are essentially protein shells that carry genetic information. They invade a cell, release the genetic material, and make the cell produce new viruses. The cell dies after a while and the new viruses are released to infect other cells.

The flu-like symptoms are mostly caused when the virus is recognized by the body and an immune reaction starts. Normally this lasts for a few days and the virus is then eradicated.

Sometimes, however, the virus stays in the body and important organs like the kidneys, the liver and the heart start to struggle. Yellow fever gets its name from the yellowish skin color that patients have because their livers are too dysfunctional to process a yellow molecule called bilirubin which, then, accumulates in the body.  The liver is also supposed to produce substances that help with blood clotting. When these aren’t around, patients can experience internal bleeding and their vomit can be black due to the blood in it.

When our body tries to fight a virus, molecules called cytokines are released that tell white blood cells to travel to the infected site. The cytokines also activate these cells to release even more cytokines. Normally the body can keep this process under control, but sometimes too many white blood cells are activated in the same place. The large amounts of cells can, for example, block airways in the lungs, and this process called a cytokine storm is one of the main causes of death in yellow fever along with organ failure.

How can it be treated?

There is an effective vaccination against yellow fever. Vaccines work through essentially showing your immune system parts of the virus, so that it can develop specific proteins that bind to the virus and make it easier for white cells to destroy it. Normally it takes time to come up with just the right protein, which is why many viruses and bacteria get to run free for a few days causing symptoms before the immune system is able to get rid of them. If you are vaccinated your immune system has those proteins in store, and when the virus tries to invade you, it is recognized straight away and it is eradicated before it has the chance to do much damage.

Once the yellow fever virus has infected the body there isn’t much to be done. Keeping the patient hydrated and managing pain with painkillers can be helpful.




Sources: https://en.wikipedia.org/wiki/Yellow_fever, http://www.who.int/csr/disease/yellowfev/en/, http://emedicine.medscape.com/article/970016-overview?pa=K8jz3g%2FK%2Fao4HVJ3ruYHW6sDa8YJFeqJawYOVr1fKiFkAL5FWdD04Xp9JDC0F06tJyGvMX%2Fu%2BWdIXoARf%2FT0zw%3D%3D#a3, https://microbewiki.kenyon.edu/index.php/Yellow_fever_virus#Pathogenesis, http://www.encyclopedia.com/medicine/diseases-and-conditions/pathology/yellow-fever, http://emedicine.medscape.com/article/232244-overview#a5, https://en.wikipedia.org/wiki/Cytokine_storm



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