A third case of swine flu in the US this year was confirmed in an unnamed patient who had recently come into contact with pigs at a fair.

The Centers for Disease Control and Prevention announced the latest infection in a tweet on Friday, but did not reveal where the person lived and attended the fair, his gender or age.

Swine flu, or H1N1, rarely spreads from animal to person, but the infamous 2009 outbreak resulted from the virus mutating, allowing people to become ill.

People can contract swine flu through contact with infected pigs, as was likely the case with the last reported infection, although this is relatively uncommon.

The symptoms of swine flu are similar to those of other influenza viruses and include fever, aches, chills, cough, headache, nausea and fatigue. But the cases are normally mild and go away on their own within a few weeks, with little risk of death.

The patient contracted the virus last month at an agricultural fair, where they were exposed to infected pigs

The driver of the 2009 swine flu epidemic was a variant of the H1N1 virus that combined avian, swine and human influenza A viruses.

The outbreak disproportionately affected children and teenagers who were more susceptible to illnesses severe enough to require hospitalization.

A World Health Organization report shows that the number of infections in the US reached 59 million in 2009, with 265,000 hospitalized and 12,000 deaths.

The latest case of three this year is worrying because it opens the door to possible human-to-human transmission.

But the speed at which H1N1 cases have emerged this year pales in comparison to the 2009 crisis, which became a global health problem within about four weeks of the variant first being discovered in Mexico.

The most recent cases of swine flu were reported in Michigan, where two unrelated people contracted different strains at different fairs in July, where they were exposed to infected pigs.

Both experienced mild illness and made full recoveries with no evidence that they had transmitted the infections to others.

Details in the latest case are scarce, but the CDC has identified several recommendations for other people to prevent possible infections when they are at an agricultural fair.

They include avoiding pigs if a person is already susceptible to serious illness, not taking food or drink into areas with pigs, washing hands before and after contact, and monitoring your pig (if you have one) for illness .

Swine flu infection from pigs to humans is relatively rare and those who have regular direct contact with pigs, such as farmers and abattoir staff, are at greatest risk of direct ‘zoonotic’ transmission.