A NSW man in his 50s is the third person to contract monkeypox in the state after recently returning from Europe.

The man developed a mild illness several days after arriving back in Sydney, NSW Health said on Friday.

He is isolating at home and his case is not connected to two earlier cases detected in NSW.

One of those cases is another NSW man in his 50s who recently returned to Sydney from Queensland, and whose case was confirmed on Thursday. He is being treated in hospital.

The viral disease can be transmitted from person to person through air droplets, close bodily contact or sharing contaminated linens or objects.

Symptoms include a fever, headache, muscle aches and exhaustion.

NSW chief health officer Kerry Chant tempered worries about the spread of the disease.

“Monkeypox does not present a transmission risk to the general community, and has until recently not been an infection most clinicians in NSW would have been looking for or concerned about in their patients,” she said.

The World Health Organisation said 23 countries that were not endemic for monkeypox virus had reported cases since mid-May. More than 640 cases have so far been confirmed.

Community health charity ACON has advised men who have sex with men to be vigilant for symptoms.

“It’s important we stay informed and continue to be self-aware when it comes to our health,” chief executive Nicholas Parkhill said.

“We know that people in our communities already have strong health-seeking behaviour when it comes to looking after their sexual health so please continue to monitor for symptoms, including for any unusual rashes or lesions.”

Mr Parkhill advised that people keep contact details for their sexual contacts, particularly amid the presence of monkeypox.

“The situation is evolving and being able to reach your sexual contacts should the need arises will help stop the spread,” Mr Parkhill said.

-with AAP