BALTIMORE (WJZ) — The Maryland Department of Health announced Wednesday that an adult living in Baltimore is the first confirmed case of West Nile Virus in the state in 2021.

The department said West Nile virus is transmitted to humans by mosquitoes who have been infected by feeding on birds who have the virus. In rare instances, the virus may be spread from person to person through organ donation, blood transfusion, breastfeeding, or from pregnant mother to fetus.

READ MORE: 'The School Shouldn't Be Open Right Now': Parents React To COVID-19 Outbreak At Cherry Hill Elementary Middle School

The disease affects the nervous system, and up to 80 percent of people who are infected will not display any signs of illness at all. Those who have underlying health conditions, however, could become seriously ill, officials said.

READ MORE: Almost 9,000 Vaccinated Marylanders Get Additional Shots Since Approval of Pfizer Booster

“This is the season when we start to see West Nile virus spread in Maryland, so we urge people to be vigilant and take steps to avoid infection,” said MDH Deputy Secretary for Public Health, Dr. Jinlene Chan. “Disease surveillance teams are closely monitoring increasing numbers of infected mosquitoes that have turned up in several areas across the state.”

The number of people testing positive for West Nile Virus varies from year. Officials said in 2018, 45 people tested positive while in 2019, only seven people were known to have contracted the virus.

MORE NEWS: 'We're The Cure To This Situation': 9 Killed, 13 Wounded In Baltimore Over The Past Week

For more information on West Nile virus, visit PHPA West Nile virus. You may also visit the Centers for Disease Control and Prevention (CDC) at https://www.cdc.gov/westnile/index.html.

CBS Baltimore Staff