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Infectious Disease

Infectious diseases are illnesses caused by germs (such as bacteria, viruses, fungi or parasites) that enter the body, multiply and can cause infection, disease, disability and even death. Some infectious diseases are vaccine preventable, whereas others are not. Infectious diseases can cause short-term, acute illness, while others can lead to chronic health problems that can be financially burdensome, personally taxing and lower the quality of life for an individual.


Examples of infectious diseases include those transmitted from human to human, from animals or other vectors (e.g., infected ticks or mosquitoes) to humans, and from contaminated food or water to humans. Infectious diseases can be communicable (e.g., STDs, Tuberculosis), meaning they pass from person to person, or non-communicable, meaning the disease is not passed from person to person (e.g., legionellosis, valley fever). 

Vaccines and Immunizations 

Immunizations are a successful and cost-effective measure to protect children and adults from numerous infectious diseases such as measles, mumps, and whooping cough. These diseases can result in extended work or school absences, hospitalizations, and even death. 


The percent of Kindergarteners with all required immunizations in SLO County was 94.1% for the 2021-2022 school year, similar to the CA average of 94.0%. 


Percent of Kindergarteners with all required immunizations in SLO County 

Percent of Kindergarteners with all required immunizations in CA
Sexually Transmitted Diseases (STDs)

In SLO County, sexually transmitted diseases (STDs) are one of the most common infectious diseases. The term STD encompasses more than 25 different infections that are transmitted primarily through sexual activity. Some are bacterial, such as chlamydia, syphilis, and gonorrhea; others are viral, such as hepatitis B, herpes, human immunodeficiency virus (HIV), and human papillomavirus (HPV); and still others are parasitic, such as trichomoniasis. Their health impacts  range from asymptomatic infection to severe disease. If left untreated, STDs cause infertility, pregnancy complications and life-threatening illness.

Chlamydia continues to be the most commonly diagnosed STDs. In 2022, the County’s chlamydia incidence rate was 293 cases per 100,000 population, which is the lowest rate in the last 10 years. While chlamydia has steadily decreased, other STDs have been rising. Gonorrhea has rebounded in recent years. In addition, syphilis hit a new SLO County high of 35 cases per 100,000 population in 2022. While counts are relatively low compared to other STDs, syphilis' serious complications if left untreated and the potentially devastating impacts during pregnancy (congenital syphilis) cause the illness to be of significant public health concern in CA and locally.  

Sexually Transmitted Disease Incidence

rates per 100,000 people

Chlamydia Incidence, by Sex

rates per 100,000 people

Syphilis Incidence, by Sex

rates per 100,000 people

GONORRHEA Incidence, by Sex

rates per 100,000 people

Valley Fever

Valley Fever, otherwise known as Coccidioidomycosis or Cocci, is a disease caused by the fungus Coccidioides. The fungus grows in the dirt/soil and becomes airborne when dirt is moved via digging, wind or other activities that can create dust. The fungus is found primarily in the southwestern United States. In California, it is largely found in the Central Valley and Central Coast regions, with SLO County having one of the highest rates in the state. 


Community members and visitors to SLO County should be aware of the signs and symptoms of Valley Fever and talk to their health care provider if they have questions or experience symptoms that do not resolve in a few weeks.

Valley Fever CASES
Foodborne Illness

Foodborne illnesses are a common public health issue, with an estimated one in six people getting sick each year in the United States from foodborne diseases. SLO County is no exception, with foodborne pathogens being a common source of illness and outbreaks. In SLO County, there have been recent increases in foodborne illnesses compared to pre-pandemic years. For example, vibriosis, salmonellosis, cryptosporidiosis, E. coli, and campylobacteriosis are foodborne diseases that have been on the rise in SLO County.

Foodborne Illnesses

rates per 100,000 people

Hepatitis C

In SLO County, Hepatitis C incidence has been decreasing, with the decline attributed to the introduction of a Hepatitis C treatment in 2013 and improved diagnosis. Hepatitis C remains a concern in SLO County with 48 cases per 100,000 population in 2022. Hepatitis C can be coupled with other diseases like HIV and other forms of hepatitis.

Hepatitis C Incidence

rates per 100,000 people


COVID-19 is the disease caused by the SARS-CoV-2 virus. It is highly contagious and often causes respiratory symptoms that can feel like a cold, the flu, or pneumonia. Many people with COVID-19 have mild symptoms, but some people become severely ill or may go on to develop Post-COVID Conditions, also called Long COVID.

COVID-19 deaths have been on a downward trend, thanks to readily available vaccine and treatment options. The disease, which was the 4th leading cause of death in SLO County in 2021 and was responsible for over 3 deaths a day in the county in early 2021, is now responsible for approximately 3 deaths a month in mid-2023.


While deaths continue to drop, health officials continue to closely monitor hospitalizations for COVID-19 and other respiratory illnesses.

See more data on COVID-19 from the CDC or from the CA Department of Public Health.

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