The number of West Nile Virus cases rose to 1,994 as of Thursday. Nearly half of the cases are in Texas.
Bexar County has seen two deaths due to the virus.
Entomologist Molly Keck said the numbers are very concerning.
"This is the largest outbreak we've had since it was introduced to the United States in 1999," Keck said.
According to data from the Centers for Disease Control, not only are more people contracting the West Nile virus, those who do contract it are getting sicker than in the past.
The virus can attack the brain, causing neurological damage. But in the past, only a small percentage of people who got West Nile developed the more serious neurological symptoms.
Keck said, "This year, more than half of those who have come down with it have the neuro-invasive symptoms."
Hans Heidner, a professor or virology with the University of Texas-San Antonio, said the dramatic increase in cases of West Nile is likely due to a number of factors.
"It's probably multiple things. It's probably conditions that have facilitated amplification of the vector, which is the mosquito. So, just sheer numbers," Heidner said.
Heidner said conditions could have allowed for more mosquitoes, or more birds which give mosquitoes the virus -- or both.
He said another possibility researchers will be looking at is whether or not they're dealing with a different, stronger strain.
"People will be doing those studies. We can't at this point say that's what's happening," Heidner said.
Whatever the cause, prevention is the most important step residents can and should take.
That begins before you walk out the door, with mosquito repellent.