The UK and the US have exposed "malicious cyber campaigns" by rival states against universities, pharmaceutical companies and other healthcare organisations involved in the coronavirus response.
Laboratories doing research on
COVID-19
vaccines are also among those being targeted.
The joint alert does not name countries behind the attacks, but they are understood to include China, Russia and Iran, as well as others.
There has not been a known successful attack on a UK institute.
Image:Staff have been advised to strengthen their passwords
Staff at healthcare and medical research organisations were on Tuesday advised to change any passwords that could be reasonably guessed.
It comes after the UK's National Cyber Security Centre (NCSC) and US Cybersecurity and Infrastructure Security Agency (CISA) discovered large-scale "password spraying" campaigns.
This is the term given to attempts to access a large number of accounts using commonly-known passwords.
Paul Chichester, the NCSC's director of operations, said: "Protecting the healthcare sector is the NCSC's first and foremost priority at this time, and we're working closely with the NHS to keep their systems safe.
"By prioritising any requests for support from health organisations and remaining in close contact with industries involved in the coronavirus response, we can inform them of any malicious activity and take the necessary steps to help them defend against it.
"But we can't do this alone, and we recommend healthcare policy makers and researchers take our actionable steps to defend themselves from password spraying campaigns."
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Staff at healthcare and medical research organisations have been told to create passwords with three random words and implement two-factor authentication to reduce the threat of compromises.
Byran Ware, CISA's assistant director of cybersecurity, said: "The trusted and continuous cybersecurity collaboration CISA has with NCSC and industry partners plays a critical role in protecting the public and organisations, specifically during this time as healthcare organisations are working at maximum capacity."
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The NCSC and CISA previously issued a joint alert on 8 April about cyber criminals exploiting the coronavirus outbreak for their own personal gain.
They expect the frequency of coronavirus-related cyber attacks to increase over the coming weeks and months.
The NCSC created a suspicious email reporting service last month after seeing an increase in coronavirus-related email scams.
In its first week, the service received more than 25,000 reports - resulting in 395 phishing sites being taken down.