A chemical found in the dogfish shark could be a safe and potent weapon against human viruses, say scientists.
Noting how powerful the shark's natural immunity to viral infections is, the researchers set about finding out why.
They already knew that the fish makes a compound called squalamine that it uses to fighting off bacteria.
Lab tests revealed squalamine is also a good antiviral candidate, killing a broad spectrum of human and animal viruses, PNAS journal reports.
Synthetic squalamine has already been given to patients in clinical trials to stop blood vessel growth in cancers, with no major side effects.
Given its safety profile and how easy it is to make, it could quickly be tested as a potential new treatment for viral diseases ranging from dengue and yellow fever to hepatitis, say the US investigators from Georgetown University Medical Center in Washington.
Their lab work shows squalamine disrupts the membrane interactions needed for viral replication.
In tissue cultures, squalamine was shown to inhibit the infection of blood vessel cells by the dengue virus, and human liver cells by hepatitis B and D.
Animal studies showed that squalamine controlled infections of yellow fever, Eastern equine encephalitis virus, and murine cytomegalovirus. In some cases, the animals were cured.
Lead researcher Prof Michael Zasloff said: "I was interested in sharks because of their seemingly primitive but effective immune system.
"No-one could explain why the shark was so hardy."
He said that realising squalamine potentially has broad antiviral properties was "immensely exciting".
"Squalamine appears to protect against viruses that attack the liver and blood tissues, and other similar compounds that we know exist in the shark likely protect against respiratory viral infections, and so on.
"We may be able to harness the shark's novel immune system to turn all of these antiviral compounds into agents that protect humans against a wide variety of viruses. That would be revolutionary.
"While many antibacterial agents exist, doctors have few antiviral drugs to help their patients, and few of those are broadly active."
He said much more work was now needed to test this new drug candidate.