Emerging Communicable Diseases

Updated February 2, 2024

General information and resources for current communicable disease topics.


Coronavirus (COVID-19) comes from a large family of viruses that are a common cause of mild upper respiratory infections, but also include both SARS and MERS, which can cause more serious disease. Most patients with coronavirus have mild symptoms, however this new virus has resulted in cases of severe illnesses and deaths reported in China.


Ebola spreads from person-to-person by direct contact with a patient’s body fluids, like saliva, blood, vomit, urine, feces, and sweat. The virus gets into the body through broken skin or mucous membranes (spongy skin like the kind you find in your nose or mouth). Ebola can also be spread by infected objects, like needles, that have been tainted with body fluids. Ebola can also spread after death, when preparing the patient’s body for burial. Ebola cannot spread through the air, in food, or water. It takes 8 - 10 days for most people to get symptoms, but it can range from 2- 21 days. Patients can spread the virus while they have a fever or other symptoms. People who do not have symptoms cannot spread Ebola.


Gastroenteritis means inflammation of the stomach and small and large intestines. Gastroenteritis is an infection caused by a variety of viruses, bacteria and parasites that results in vomiting or diarrhea. Gastroenteritis is associated with foodborne illnesses via food contamination.

Food contamination can occur in the following ways:

  1. Natural bacteria found in food are allowed to grow to harmful levels. This can happen when food is cooked or stored at the wrong temperature.
  2. The cooking area is not kept clean. For example, when insects or rodents get into the kitchen area.
  3. The cooking utensils are not cleaned properly. For example, a board used to cut raw meat is then used for foods eaten raw. This is called cross-contamination.

Bacteria may also grow on wet, contaminated cloths and sponges (water may get rid of dirt you can see, but not bacteria).

Many times foodborne illness goes unrecognized, or is thought to simply be a "stomach flu." Symptoms may occur within a few hours to days after eating contaminated food. It is often not the last thing eaten that makes a person sick. Symptoms may include stomach cramps, diarrheas, fever, nausea, vomiting and chills.

Gastroenteritis is highly infectious and is spread by the vomit or faeces of an infected person through person-to-person contact, for example shaking hands with someone who has been sick and has the virus on their hands, contaminated objects and contaminated food or drink. Infection may also be spread through aerosolised particles when people vomit. In most cases, spread occurs from a person who has symptoms. Some people can pass on the infection without symptoms, particularly in the first 48 hours after recovery.

Hepatitis A

Hepatitis A viral infection causes liver disease that can range from mild to severe, and for some will result in death. The virus is spread via the fecal-oral route, i.e. the virus is ingested by mouth from contact with hands, objects, food or drinks that are contaminated by the feces of an infected person. Regular hand washing, with soap and water is an effective prevention strategy. Person-to-person transmission through the fecal-oral route (i.e., ingestion of something that has been contaminated with the feces of an infected person) is the primary means of HAV transmission in the United States. Exposure to contaminated food or water can cause common-source outbreaks and sporadic cases of HAV infection. Uncooked foods contaminated with HAV can be a source of outbreaks, as well as cooked foods that are not heated to temperatures capable of killing the virus during preparation and foods that are contaminated after cooking, as occurs in outbreaks associated with infected food handlers.


Influenza, or flu, is a respiratory illness caused by influenza viruses. Transmission of influenza in humans can occur via respiratory infection by aerosols and droplets (from coughing and sneezing) or from contact transmission from contaminated surfaces. Closed environment and crowds favor transmission. Transmission of influenza virus from donors who are shedding large amounts of virus can be infective for 2 to 8 hours. Symptoms of influenza include fever (often high), runny or stuffy nose, cough, sore throat, headache, tiredness, and muscle aches. Children may sometimes also have nausea, vomiting and diarrhea. Some people can have a severe illness with influenza. They may have pneumonia, ear infections, dehydration, and worsening of medical problems such as asthma, heart conditions, and diabetes. People with chronic medical conditions, the elderly, pregnant women, and children under 2 years of age are more likely to be admitted to a hospital for influenza. More than 200,000 people are hospitalized each year in the United States due to flu and between about 3,000 to 49,000 people die.


Measles is a highly contagious and potentially severe disease that causes fever, rash, cough, and red, watery eyes. Measles spreads very easily by air and by direct contact with an infected person. Measles is contagious from approximately 4 days before the rash appears through 4 days after the rash appears. Measles is a rare disease in the United States and in regions of the world where vaccination coverage is high. Maintaining high vaccination rates is vital to preventing outbreaks of disease in our community. Mumps is best known for the puffy cheeks and swollen jaw that it causes. This is a result of swollen salivary glands. The most common symptoms include: fever, headache, muscle aches, tiredness, loss of appetite, swollen and tender salivary glands under the ears on one or both sides (parotitis). Symptoms typically appear 16-18 days after infection, but this period can range from 12-25 days after infection. Some people who get mumps have very mild or no symptoms, and often they do not know they have the disease. Most people with mumps recover completely in a few weeks. Mumps is a contagious disease caused by a virus. It spreads through saliva or mucus from the mouth, nose, or throat. An infected person can spread the virus by coughing, sneezing, or talking, sharing items, such as cups or eating utensils, with others, and touching objects or surfaces with unwashed hands that are then touched by others. Mumps likely spreads before the salivary glands begin to swell and up to five days after the swelling begins.


Meningitis is an inflammation (swelling) of the protective membranes covering the brain and spinal cord. A bacterial or viral infection of the fluid surrounding the brain and spinal cord usually causes the swelling. However, injuries, cancer, certain drugs, and other types of infections also can cause meningitis. It is important to know the specific cause of meningitis because the treatment differs depending on the cause. Meningitis can be caused by bacteria, virus, fungi, parasites. Meningitis is not easily spread like the common cold or flu. Because it is spread through close contact, avoid sharing drinks, water bottles, lip-balm, or smoking materials. Cover all coughs and sneezes, and wash your hands thoroughly.


Monkeypox is a rare disease caused by infection with the monkeypox virus. Monkeypox is part of the same family of viruses that cause smallpox. Monkeypox symptoms are similar to smallpox symptoms, but milder, and monkeypox is rarely fatal. Monkeypox might start with symptoms like the flu, with fever, low energy, swollen lymph nodes, and general body aches. Within 1 to 3 days (sometimes longer) after the appearance of fever, the person can develop a rash or sores.


Mono, or infectious mononucleosis, refers to an illness usually caused by the Epstein-Barr Virus (EBV). The virus is spread mainly through direct contact with saliva of an infected person, which is why some people refer to it as “the kissing disease.” It can also be spread through blood, sexual contact, and organ transplantation.

Staphylococcus aureus

Staphylococcus aureus (Staph) is a bacteria commonly found on the skin and in the nose of healthy people. Some Staph bacteria have developed resistance to certain antibiotics commonly used to treat Staph infections, and are called Methicillin-resistant Staphylococcus aureus, or MRSA. Staph, including MRSA, can cause minor infections such as pimples and boils, or it can cause more serious infections, such as abscesses, pneumonia, and bone or bloodstream infections. Many MRSA skin infections are initially misdiagnosed as spider bites. However, verified spider bites are extremely rare and the spiders that cause significant bites are uncommon in Southern California. Some people have MRSA on their body or in their nose but no symptoms of infection. MRSA is transmitted most frequently by direct skin-to-skin contact or contact with shared items or surfaces (e.g., towels, used bandages) that have come into contact with someone else's infected site.


Pertussis, a respiratory illness commonly known as whooping cough, is very contagious disease caused by a type of bacteria called Bordetella Pertussis. These bacteria attach to the cilia (tiny, hair-like extensions) that line part of the upper respiratory system. The bacteria release toxins (poisons), which damage the cilia and cause airways to swell. Pertussis spreads from person to person. People with pertussis usually spread the disease to another person by coughing or sneezing or when spending a lot of time near one another where you share breathing space. Many babies who get pertussis are infected by older siblings, parents, or caregivers who might not even know they have the disease. Infected people are most contagious up to about 2 weeks after the cough begins. Antibiotics may shorten the amount of time someone is contagious. While pertussis vaccines are the most effective tool to prevent this disease, no vaccine is 100% effective. When pertussis circulates in the community, there is a chance that a fully vaccinated person, of any age, can catch this disease. If you have gotten the pertussis vaccine but still get sick, the infection is usually not as bad.


Tuberculosis (TB) is caused by a bacterium called Mycobacterium tuberculosis. The bacteria usually attack the lungs, but TB bacteria can attack any part of the body such as the kidney, spine, and brain. Not everyone infected with TB bacteria becomes sick. As a result, two TB-related conditions exist: latent TB infection (LTBI) and TB disease. If not treated properly, TB disease can be fatal. TB bacteria are spread through the air from one person to another. The TB bacteria are put into the air when a person with TB disease of the lungs or throat coughs, speaks, or sings. People nearby may breathe in these bacteria and become infected. TB is not spread by shaking someone’s hand, sharing food or drink, touching bed linens or toilet seats, sharing toothbrushes and kissing. When a person breathes in TB bacteria, the bacteria can settle in the lungs and begin to grow. From there, they can move through the blood to other parts of the body, such as the kidney, spine, and brain. TB disease in the lungs or throat can be infectious. This means that the bacteria can be spread to other people. TB in other parts of the body, such as the kidney or spine, is usually not infectious. People with TB disease are most likely to spread it to people they spend time with every day. This includes family members, friends, and coworkers or schoolmates.

West Nile Virus

West Nile virus (WNV) is a virus that is most commonly spread by the bite of an infected mosquito. It affects the central nervous system and can cause a potentially serious illness with varying symptoms. People who spend a lot of time outdoors are more likely to be bitten by an infected mosquito, and people older than 50 years of age are at increased risk for severe disease if bitten by an infected mosquito. In a very small number of cases, WNV was spread through blood transfusions, organ transplantations, from mother-to-baby (during pregnancy and through breastfeeding), and through work exposures (animal handling or laboratory). Animals can also be infected with WNV and certain birds in particular play an important part in the life cycle and spread of the virus although birds do not directly spread the infection to humans. There is no treatment except supportive care for WNV infection, although experimental therapies are currently being studied. Avoiding mosquito bites is the #1 way to prevent WNV infection.


Zika virus is transmitted by Aedes aegypti mosquitoes (also known as yellow fever mosquitoes) and by Aedes albopictus mosquitoes (also known as Asian tiger mosquitoes). These mosquitoes are not native to California. However, since 2011 they have been detected in several California counties. An Aedes mosquito can only transmit Zika virus after it bites a person who has this virus in their blood. Thus far in California, Zika virus infections have been documented only in a few people who were infected while travelling outside the United States. A person with Zika is not contagious. Zika is not spread through casual contact such as touching or kissing a person with the virus, or by breathing in the virus.