Top US health officials say the more they learn about Zika, the scarier the virus appears.
Dr Anthony Fauci, of the National Institutes of Health, said he's "not an alarmist," but more money is needed to fight the mosquitoes that spread it - and for research into vaccines and treatments.
He cites recent discoveries about how destructive Zika appears to be to foetal brains.
There also are reports of rare neurologic problems in adults.
The Obama administration is using some leftover money from the Ebola fight to pay for Zika research, but that's just a fraction of the $1.9 billion dollars it sought from Congress.
Mr Fauci says the $589 million dollars now available is a "temporary stopgap" and it is "not enough for us to get the job done".