Medical insurance is strongly advised. Only emergency cases are treated without prior payment and treatment may be refused without evidence of insurance or a deposit. All receipts must be kept in order to make a claim. Medical facilities are generally of a high standard. Many medications available over the counter in other countries require a prescription in the US. Those visiting the USA for long periods with school-age children should be aware that school entry requirements include proof of immunisation against diphtheria, measles, poliomyelitis and rubella throughout the USA; schools in many states also require immunisation against tetanus, pertussis (whooping cough) and mumps. HIV-positive visitors must apply at the US Embassy for a Waiver of Inadmissibility before entry.