What can be called "Cat Flu" is a common name for Feline Immunovirus, which is an infectious viral disease among cats (and dogs). It is generally fatal and highly infectious. it is spread by the saliva or faeces of an infected animal, and since, in a warm country like Thailand, the virus can survive outside the animals body, it can spread quickly through a population. There is no cure for the disease. It can be prevented by annual vaccinations, given by your vet, which are virtually 100% effective. It is common in Thailand because many owners do not have their pets vaccinated. In say UK or US most pets are vaccinated annually, so there is no population of vulnerable animals to keep the disease alive. Vaccine takes 1 week to become effective, so it is too late to do once the disease strikes.
Search the net for more information, or see your local vet for which vaccinations your pet will need., starting at 3 months.