America is in the midst of its biggest ever outbreak of West Nile virus, with four times the usual number of cases reported for this time of year.

Nearly 1,120 people have already contracted the mosquito-borne illness, up from fewer than 300 by mid-August in an ordinary year, and 41 people have died.

Medical experts believe the mild winter and very hot summer boosted the number of mosquitoes around, helping to spread the virus.

But it's still too early to say how bad epidemic will end up because most West Nile infections are reported in August and September.

'We're in the midst of one of the largest West Nile outbreaks ever seen in the United States,' said Dr Lyle Petersen, of the Centers for Disease Control and Prevention.

Never before have so many cases been called in this early in the year and the number of patients are accelerating, with about 400 of them being struck down in just the last week.

Almost half of the cases so far reported have come from Texas, mainly around the Dallas area.

Twenty-one people have died in the state this year, which is more than all other years combined. Four of the Texas deaths were reported on Tuesday.

Officials don't know why Texas is seeing so many cases, but it's alarming.

Last week, they started aerial spraying for mosquitoes in Dallas County.

But it's too soon to measure the effect - it takes between three and 14 days for people to develop symptoms after being infected by a mosquito.

But infections were not restricted to Texas, with residents in 37 other states have also been struck down.

Mosquitoes pick up West Nile from birds they bite and then pass it on to people.

As well as weather conditions spurring on mosquito breeding, CDC officials are also looking into the possibility that the virus mutated, Petersen said. But so far they have no evidence of this.

The latest person to succomb to the disease was 76-year-old William Mueller from Illinois who died on Saturday, two weeks after being hospitalised.

Serving as president of Lombard Village, Mueller was described by his family as 'an amazing dad, husband and grandpa'.

'It is not clear why we are seeing more activity than in recent years,' Marc Fischer of the CDC told CNN.

'Regardless of the reasons for the increase, people should be aware of the West Nile virus activity in their area and take action to protect themselves and their family.'