While Maryland residents participate in outdoor activities this summer, the difference between contracting or not contracting West Nile virus could be a spritz of mosquito repellent.

The virus is spread by infected mosquitoes to humans and other animals.

The first case of West Nile virus in North America was recorded in 1999. Since then, there have been more than 37,000 cases and about 1,500 deaths linked to the disease nationwide, according to a report by the Arboviral Disease Branch of the Centers for Disease Control and Prevention.

About 80 percent of humans bitten by infected mosquitoes will not suffer symptoms, said Marc Fischer, an epidemiologist at the CDC. Those who are affected usually only suffer flulike symptoms. About 1 percent of West Nile virus cases result in hospitalization, he said.

The Maryland Department of Health and Mental Hygiene began this year’s annual tracking of West Nile virus cases July 1. Kimberly Mitchell, chief of rabies and vector-borne diseases at the department, said the majority of cases will start appearing late this month and early next month.

Each year, the department of health works with the Department of Agriculture and the Department of Natural Resources to combat infected mosquitoes.

The department of health is responsible for discovering and reporting cases and suspected breeding areas of infected mosquitoes. The Department of Agriculture will then spray the area with repellent, supplementing the normally scheduled sprays. The Department of Natural Resources will test the blood of animals that show signs of the disease, Mitchell said.

The department of health depends on residents to report cases of West Nile virus. The department records the data, analyzes it to see which areas of the state are more prone to the disease and submits it to the CDC. The analyzed data are used to predict how the disease will spread in coming weeks.

Fischer said the CDC uses its aggregated data to make the same type of predictions for a national scale.

The annually collected data help provide the health department with information about the pattern and epidemiology of the virus, Mitchell said. There is no cure for the disease; doctors can only prescribe general symptom-reducing medicines.

But residents can stop West Nile virus from affecting them with simple habits.

West Nile virus “is preventable through behavior,” Mitchell said.

Fischer suggests always using insect repellent when participating in outdoor activities and wearing long-sleeve shirts and pants when possible, especially outside at night, which is prime mosquito-biting time.

“They tend to bite during evenings and mornings,” Fischer added.

Residents also can protect themselves by using screens on their doors and windows, using air conditioning when possible and getting rid of standing water baths, where mosquitoes tend to breed.

These precautions are necessary because the severity and number of West Nile virus cases that will occur in Maryland and across the country this year is unpredictable, Fischer said.

The department of health began its West Nile virus surveillance program in 2000. Since then, the year with the highest number of human cases was 2003, when 73 were recorded in the state and almost 10,000 across the nation.

From 2004 through 2011, the number of state cases never rose above 25. Last year, the count reached 47 cases in Maryland and almost 6,000 nationwide, the second-highest numbers on record.“The number of cases increases and decreases from year to year and place to place,” Fischer said.Once cases are recorded in the state, a weekly report will be available to the public on the department of health’s Web site, http://phpa.dhmh.maryland.gov .

Information about reporting a potential West Nile virus case can also be found at the Web site.