Anyone who coughs on key workers as a threat amid the coronavirus crisis will face serious criminal charges.
The warning from the Director of Public Prosecutions for England and Wales comes after reports of essential workers being coughed at by people claiming they have the virus.
Max Hill QC said he was "appalled" by the incidents and the full force of the law would be used.
Two men in England have already been convicted - one of them jailed.
The warning came as the government announced tough new restrictions on people who breach coronavirus lockdown rules.
Coughs or spits directed at key workers - or threats to do so - can be considered crimes if they were meant to harm or cause fear, with criminals charged with common assault.
In England and Wales, common assault can lead to six months in prison - and attacks against emergency workers going about their duties carry a maximum sentence of two years.
Mr Hill said: "Emergency workers are more essential than ever as society comes together to tackle the coronavirus pandemic.
"I am therefore appalled by reports of police officers and other frontline workers being deliberately coughed at by people claiming to have Covid-19."
"Let me be very clear: this is a crime and needs to stop. The Crown Prosecution Service stands behind emergency and essential workers and will not hesitate to prosecute anybody who threatens them as they go about their vital duties."
On Tuesday, David Mott, 40, spat at a Lancashire Police sergeant, saying he wanted to give her Covid-19.
Officers had been directing him and others to go home under the government's guidance to stay indoors.
Blackburn magistrates jailed him for 26 weeks yesterday for the threats and other offences.
Also on Wednesday, Darren Rafferty, 45, of Dagenham, east London, pleaded guilty to grievous bodily harm against his former partner and three counts of assaulting an emergency worker.
During the incident, Rafferty coughed at Metropolitan Police officers, saying he was infected with Covid-19. He was remanded in custody ahead of sentencing on 1 April.
A 39-year-old man who claimed to have Covid-19 is to appear in court in Belfast on Thursday, after he too allegedly coughed towards two police officers.
John Apter, chair of the Police Federation of England and Wales, which represents rank-and-file officers, said he had spoken this week with Home Secretary Priti Patel about such "vile" threats.
"I wanted emergency legislation brought in to offer increased protection for all 999 workers who are being attacked in this way. There are offences already available but they are generally minor and do not attract the level of seriousness they deserve.
"In our campaign to see better protection for 999 workers, I have often been a critic of the sometimes soft sentences dished out to those who assault my colleagues and those from other emergency services. In this time of crisis the comments from Max Hill are welcome and timely."
The CPS says it has prosecuted almost 20,000 assaults against emergency workers since legislation first came into force in November 2018.