Yellow fever is a virus that is transmitted to humans by particular species of mosquitoes -- in North American outbreaks, the Aegis Egyptae.

Female mosquitoes with infected parents can be born with the disease, or they can feed on infected humans, acquiring the disease along with the human blood.

The virus replicates in the mosquito's digestive tract, and is then carried through the bloodstream to the salivary glands. This cycle takes between seven and seventeen days, during which the mosquito can not transmit the virus.

After the virus has spread to the salivary glands of the insect, the next time the mosquito feeds, she will transmit the virus into the human skin. From the skin, the virus multiplies and is carried into the lymph nodes, which carries it into the bloodstream. Then symptoms appear.

Patients who survive yellow fever develop immunity from further infections.

Mosquitoes will not survive a frost. However, infected female mosquitoes pass yellow fever to their eggs. These eggs can lie dormant through dry, winter conditions, hatching the next year, releasing new infected mosquitoes; this is the reason the disease returned annually. Areas of standing water are prime breeding grounds and are emblematic of outbreak areas.

Bananas are unloaded at a busy New Orleans port.

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