Aid workers in Tonga warn the tropical cyclone set to hit the South Pacific nation for the second time this week could help spread the Zika virus.
Tropical Cyclone Winston is expected to intensify to a category four storm, the second highest level, when it again strikes Tonga's northern islands later tonight, according to Tonga Meteorological Services.
It said it expected winds to reach as high as 95-110km/h. Heavy rain is also forecast.
Aid agencies said they were concerned cyclone Winston could cause severe damage after it first hit northern islands earlier this week, especially after a prolonged El Nino-induced drought weakened livestock and crops across Tonga.
Heavy rain could also bring another severe impact even after the storm passed, aid agencies warn.
"Another deadly threat lies in the wake of Winston: the presence of Zika virus in Tonga could spread easily in a response situation," said Carlos Calderon, the Pacific humanitarian manager for Oxfam New Zealand.
Oxfam said cyclone Winston could disrupt access to running drinking water and hygienic drainage systems or mosquito control measures such as nets, raising the risk of an increase in mosquito breeding and a fresh outbreak of the Zika virus.
Aid workers have begun removing mosquito larvae from water tanks, spraying affected areas to remove the threat of Zika, and distributing mosquito nets, with a focus on pregnant women.
Tonga declared a Zika outbreak in February after five cases of the mosquito-borne illness were confirmed and another 259 suspected.
The World Health Organisation declared the outbreak a global public health emergency on February 1, noting its association with two neurological disorders: microcephaly in babies and Guillain-Barre syndrome, which can cause paralysis.