Ans. is 'a' i.e., 1% For the surveillance of Aedes mosquitoes, the WHO uses an index known as Aedes aegypti index. This is a house index and is defined as "The percentage of houses and their premises, in a limited well-defined area, showing actual breeding of Aedes aegypti larvae". This index should not be more than 1% in towns and seapos in endeic areas to ensure freedom from yellow fever. International health regulation for yellow fever Measures designed to restrict the spread of yellow fever are specified in the "International health regulation" of WHO. These are implemented by the Govt of India through stringent aerial and maritime traffic regulations. Broadly these comprise : - i) Travellers All travellers (including infants) exposed to yellow fever or passing through endemic zones of yellow fever must posses a valid international ceificate fo vaccination against yellow fever before they are allowed to enter yellow fever receptive areas like India. The validity of the ceificate begins 10 days after the date of vaccination and extends up to 10 years. Revaccination performed before the end of the validity of ceicicate renders the ceificate valid for a fuher period of 10 years staing on the day of revaccination. If no such ceificate fo vaccination is available, the travellar is placed on quarantine for 6 days from the date of leaving an infected area. ii) Mosquitoes The aircraft and ships arriving from endemic areas are subjected to aerosol spraying with prescribed insecticides. Airpos and Seapos are kept free from the breeding of insect vectors over an area extending at least 400 metres around their perimeters. The "aedes aegypti index" is kept below 1.