A staff member from local health department spreads pest-control chemicals to kill mosquitoes and larvae to prevent mosquito-borne diseases in Kolkata, India, on Sep. 2, 2019. (Photo: Xinhua)

A staff member from local health department spreads pest-control chemicals to kill mosquitoes and larvae to prevent mosquito-borne diseases in Kolkata, India, on Sep. 2, 2019. (Photo: Xinhua)

Staff members from local health department fumigate a settlement to kill mosquitoes and larvae to prevent mosquito-borne diseases in Kolkata, India, on Sep. 2, 2019. (Photo: Xinhua)

A staff member from local health department fumigates a settlement to kill mosquitoes and larvae to prevent mosquito-borne diseases in Kolkata, India, on Sep. 2, 2019. (Photo: Xinhua)

A staff member from local health department fumigates a settlement to kill mosquitoes and larvae to prevent mosquito-borne diseases in Kolkata, India, on Sep. 2, 2019. (Photo: Xinhua)

A staff member from local health department fumigates a settlement to kill mosquitoes and larvae to prevent mosquito-borne diseases in Kolkata, India, on Sep. 2, 2019. (Photo: Xinhua)