Mosquitoes carry the highest amounts of the virus in early autumn, so there is a maximum incidence of the disease in late August and early September. The risk of suffering from it then decreases as the climate becomes colder and the mosquitoes begin to become extinct.
Although many people are bitten by mosquitoes carrying West Nile virus, most do not know they have been infected.
Possible risk factors for contracting a more serious type of the disease include the following:

• Conditions that weaken the immune system, such as HIV, organ transplant, and recent chemotherapy 
• Old or very young people
• Pregnancy

West Nile virus can also be spread through blood transfusions and organ transplants. It is also possible for an infected mother to transmit the virus to her child through breast milk.

Symptoms:

Symptoms can occur 1 to 14 days after becoming infected. Mild disease, usually called West Nile fever, can cause some or all of the following symptoms:

• Abdominal pain
• Diarrhea
• Fever
• Headache
• Inappetence
• Muscle pains
• Sickness
• Rash
• Sore throat
• Lymph nodes inflammation
• Vomiting

These symptoms usually last 3 to 6 days, but may last a month

The most severe forms of the disease, which are life-threatening, can be called encephalitis or West Nile meningitis, depending on which part of the body is affected.