(CNN) An outbreak of hepatitis A has sickened 421 people and killed 16 in San Diego County between November and September, officials say. Additionally, 292 of the 421 have been hospitalized as a result of the viral infection, which causes inflammation of the liver.
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.
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.
"The homeless community was very thankful and appreciative. We're looking at other locations where they can be deployed."
"We're also encouraging municipalities to look at other things they can do, like increasing access to public bathrooms," Wooten said, adding that bleach is needed to kill the virus when cleaning restrooms.
"The city of San Diego is also looking at mobile showers and also strategies to help people wash their clothes," she said. "All of those things have been considered to help with personal hygiene as well as sanitation of the streets."
Craig Gustafson, senior director of communications for the city of San Diego, wrote in an email that "the City hired a vendor to begin using a bleach cleaning solution on public right-of-ways on Monday. The cleanings will occur three times a week (Mondays, Wednesdays and Fridays) every other week in the downtown area."
The education outreach includes flyers posted and pamphlets available in public places, including municipal buildings and libraries.
The state of emergency "could lead to additional state and federal funding to reimburse for costs related the outbreak," Gustafson said.
Separately, Santa Cruz County "has reported 69 cases in 2017," Jorge De La Cruz, a spokesman for the California Department of Public Health, wrote in an email. State public health officials consider this an additional outbreak.
Two challenges involved in the investigation of the Santa Cruz outbreak, according to the county health department website, are the fact that hepatitis A has a long incubation period -- 15 to 50 days -- and that many infected people have been difficult to contact due to homelessness or illicit drug use.
"To date, no common source of food, beverage, or other cause has been identified; as a result, the source of the outbreak remains undetermined," the website says.
"Outside of San Diego and Santa Cruz counties, 126 cases of hepatitis A have been reported to date (in California) in 2017," De La Cruz wrote, adding that the total number of cases in the state as of Thursday was 631.
On Tuesday, the Los Angeles County Department of Public Health said it has confirmed 10 cases of hepatitis A among homeless people.
Dr. Jeffrey Gunzenhauser, the county's interim health officer, said in a statement, "our outreach teams and clinics are offering free vaccine to persons who are homeless, active drug users, and those who provide services and support to those individuals." The most recent cases in Los Angeles appear to be locally acquired, according to the department.
Wooten said that "underlying medical conditions like hepatitis B or hepatitis C or both, or other chronic liver disease" may be contributing to the high hospitalization and death rates among the homeless in San Diego.
"Going forward, we are doing everything we can do and working with all of our partners, and I can't predict how long this will last. We will be working on this anywhere from another six to 12 months, potentially," she said. "If we get our arms around it sooner, we welcome that."