The first human cases of West Nile virus have appeared in Minnesota this summer. The disease is carried by a certain breed of mosquito, and it's around this time of the year that West Nile cases begin popping up.

The Minnesota Department of Health says West Nile was found in four Minnesota residents who donated blood recently, according to WCCO.

West Nile virus is usually detected in mosquito samples in July, and it circulates among mosquitoes and birds. Then later, those mosquitos bite humans and can spread the virus.

The return of the virus – which causes flu-like symptoms and can in very rare cases be fatal – happened at a similar time as last year.

Last year, four people in the state contracted the disease, according to the state health department.

But in 2013, the numbers were far higher - 79 human cases of the virus were reported across 39 counties, and three of those people died.

That variation from year to year is pretty typical, according to Mike McLean of the Metropolitan Mosquito Control District.

“You can have anywhere from two or three cases in the whole state to 100, 150 cases in the whole state, so very difficult to predict,” McLean told WCCO.

McLean added that because it hasn't been too hot this summer, the number of West Nile cases is likely to be be relatively low.

How does West Nile affect humans?

The MDH says most people infected with the virus won’t notice any symptoms at all, but about one in five will develop symptoms of West Nile fever, which can include a high fever, severe headache, nausea, vomiting, sore throat, a rash, swollen lymph nodes, backache and joint pain.

Around one in 150 people will develop West Nile encephalitis, a more severe neurological disorder that causes inflammation of the brain. The Centers for Disease Control and Prevention (CDC) says 10 percent of people who get West Nile encephalitis die.

The MMCD is asking people to report any dead birds they see (the West Nile virus can be fatal to certain species of birds) to help them track the virus’ spread across the state.

They also suggest people reduce their exposure to mosquitoes by:

• Clearing water-holding containers in yards or neighborhoods (where mosquitoes can develop).
• Ensuring window screens are in good repair.
• Wearing long-sleeved, loose-fitting, light-colored clothing outside.
• Using bug spray.
• Avoiding prolonged outdoor activity at dawn and dusk – when mosquitoes are at their most active.