Officials in South Africa, the host of the 2010 World Cup from June 11 to July 11, have reported 172 human cases of the animal viral disease this year, including 15 deaths, it said.
Many tourists visit South Africa's game parks and the WHO warned visitors to avoid contact with dead animals - another way of catching the disease.
"People should be aware there is Rift Valley Fever in several provinces and should take precautions when visiting areas where they could be exposed to animals that could be infected," WHO spokeswoman Aphaluck Bhatiasevi told Reuters.
A female German tourist who visited game reserves was confirmed as having the disease upon return from South Africa last month, the WHO said in a statement. Three other travellers fell ill with similar symptoms but all four have recovered.
The virus usually causes relatively mild flu-like symptoms and neck stiffness in people, progressing sometimes to hallucinations, dizziness or even coma, according to the WHO.
A small percentage of victims develop a haemorrhagic form which causes them to vomit blood, pass blood in faeces, or bleed from the nose, gums, or skin. Half of such patients die.
Human cases have been confirmed in Free State Province, Eastern Cape Province, Northern Cape Province, Western Cape and North West Province, most after contact with infected livestock.
MOSQUITO BITES
The WHO, a United Nations agency, said it was not advising any international travel restrictions to or from South Africa.
"However, WHO recommends that visitors to South Africa, especially those intending to visit farms and/or game reserves, avoid coming into contact with animal tissues or blood, avoid drinking unpasteurised or uncooked milk or eating raw meat."
"All travellers should take appropriate precautions against mosquito bites (use of mosquito nets, insect repellents)," it said.
Most human infections result from contact with the blood or organs of infected animals, according to the WHO.
It can also be caught from the bites of infected mosquitoes or other flies.
There is also some evidence that humans may become infected with Rift Valley Fever by drinking the unpasteurised or uncooked milk of infected animals, it said.
The virus can be transmitted through the handling of animal tissue during slaughtering or butchering, assisting with animal births, veterinary procedures or from disposing of carcasses.
Herders, farmers, slaughterhouse workers and veterinarians are at higher risk of infection.
A vaccine for humans has been developed but is not licensed and is not commercially available, according to the WHO.
