西尼罗河病毒是一种热带和温带地区病毒。它主要传染鸟，但还传染人、马、猫、臭鼬、灰鼠，和家养的兔子。人的传染主路是由被传染的蚊子的叮咬。

大约80%的人类感染后没有明显症状。20%左右的人只有轻微的类似流感症状，如倦怠、发热、不思饮食、恶心、眼部、头和肌肉疼痛、皮疹和淋巴结肿大,此称为西尼罗河热。

在0.7%的感染者中，病毒通过血脑屏障进入大脑后导致脑膜炎或脑炎（西尼罗河脑炎），这两类感染可导致死亡，对50岁以上的患者威胁较大。这些人在感染后3-15天出现症状，包括颈部僵硬，神智异常，抽筋、昏迷，偶尔有失明现象，持续时间为3-6天。目前这两类疾病可以用检验血液和脑脊液的抗体来诊断，但目前除了住院对症治疗外，没有特别的医治方法。在这些少数神经受伤的疾病患者可能因此病毒而终身造成神经伤害。

West Nile virus (WNV) is a mosquito-borne arbovirus of the Flavivirus kind in the family Flaviviridae. Approximately 80% of West Nile virus infections in humans are subclinical, which cause no symptoms. In the cases where symptoms do occur—termed West Nile fever in cases without neurological disease—the time from infection to the appearance of symptoms (incubation period) is typically between 2 and 15 days. Symptoms may include fever, headaches, fatigue, muscle pain or aches (myalgias), malaise, nausea, anorexia, vomiting, and rash. Less than 1% of the cases are severe and result in neurological disease when the central nervous system is affected. People of advanced age, the very young, or those with immunosuppression, either medically induced, such as those taking immunosupressive drugs, or due to a pre-existing medical condition such as HIV infection, are most susceptible. The specific neurological diseases that may occur are West Nile encephalitis, which causes inflammation of the brain, West Nile meningitis, which causes inflammation of the meninges, which are the protective membranes that cover the brain and spinal cord, West Nile meningoencephalitis, which causes inflammation of the brain and also the meninges surrounding it, and West Nile poliomyelitis—spinal cord inflammation, which results in a syndrome similar to polio, which may cause acute flaccid paralysis.