The World Health Organization says a feared second wave of deaths from Pakistan's flood has so far failed to materialise. But a WHO representative in Pakistan says the ''risk is very high'' that waterborne diseases such as cholera could spread and cause large numbers of deaths. Dr. Guido Sabatinelli says there have been 57 confirmed cases of the disease in recent weeks. He told reporters in Geneva by phone Friday that malaria and measles also pose an immediate risk.