Over the previous five years, the county recorded just 28 infections per year on average.
"This is an outbreak of unprecedented proportion, and we have not seen an outbreak of this nature as relates to hepatitis A before," said Dr. Wilma Wooten, public health officer and director of public health services for the county's Health and Human Services Agency.
Most of the infections, 65%, are occurring among people who are homeless, who use illicit drugs or who experience a combination of those two factors.
The remaining cases include 23% in people associated with the homeless population, such as friends, and 12% in people who are lacking records and are most likely homeless, according to Wooten.
Hepatitis A is typically spread when people eat contaminated food or water; more rarely, the virus is transmitted person-to-person through the "fecal-oral route," she explained.
"Basically, if an individual is infected with hepatitis A and they use the bathroom and don't wash their hands, and then they can spread or contaminate the environment: door handles, ATMs or whatever they touch," Wooten said. Other people who touch the same objects and fail to wash their hands before eating, smoking or touching their faces can become infected as well.
Symptoms of hepatitis A
, which include fever, fatigue, nausea and abdominal pain, usually last less than two months, although some people can be ill for as long as six months, according to the Centers for Disease Control and Prevention. Although there are no special treatments, doctors recommend rest, adequate nutrition and fluids. Most people recover completely and do not have any lasting liver damage.
Wooten declared a state of emergency for the county, home to about 3.3 million people, on September 1.
"The local emergency was declared to increase and heighten awareness of the seriousness of the outbreak," she said.
Vaccinations, bleach and more
To combat the outbreak, public health officials have adopted a three-part strategy: vaccination; sanitation, which includes hygiene; and education.
Since June, the San Diego Police Department has been offering free hepatitis A shots through its homeless outreach and other programs.
"We've vaccinated over 21,000 individuals," Wooten said. "The majority of those are the at-risk population or individuals who have association with the at-risk population."
The San Diego Central Library will host free vaccination clinics on the third Tuesday of each month beginning September 19.
The city of San Diego -- one of 18 incorporated municipalities and one unincorporated municipality that make up the county -- has "the largest number of homeless individuals downtown," Wooten said, "so we deployed or put out hand-washing stations" about two weeks ago.