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Why are a handful of measles cases an 'outbreak'?

Measles is highly contagious, and it produces fever and rash in susceptible people who become infected.

Los Angeles County's public health department said Friday that it has confirmed two more measles cases, bringing to nine the number it has identified in the outbreak that it says began a month ago. 

So why does such a small number of cases constitute an outbreak?

Public Health says it declared an outbreak because "the number of cases is definitely greater than expected and the majority of confirmed cases are epidemiologically linked."

The criteria for defining an outbreak depend on the particular disease. For "uncommon conditions," the department says, an outbreak could be "two or more cases that are connected by social, environmental or geographical circumstance." 

The agency did not clarify whether it considers measles "uncommon." The disease was officially declared eliminated in the U.S. in 2000, but there have been sporadic cases and outbreaks since then.

For "very rare" diseases, like Anthrax or Ebola, the occurrence of a single case could constitute an outbreak, according to Public Health.

An increase of cases in a given time or location might be a "cluster" rather than an outbreak, if health officials don't confirm an epidemiological link, according to the department.

In the case of an outbreak or a cluster, Public Health says it investigates to define linkages between cases, "determines the magnitude of the problem and the threat to public health, identifies risk factors for infection, and implements preventive interventions." That includes contacting and testing anyone who came in contact with those who got sick.

The department notes that when a vaccine-preventable illness such as measles is involved, the declaration of an outbreak frees up federal funds to help in controlling the spread of the disease.

In the current measles outbreak,  the first person who got sick developed symptoms in late November, the agency says.

For privacy reasons, Public Health has not revealed the patients' ages or where they were infected.

"Some exposures may have occurred in public locations," the department says, adding that it's investigating those cases. If the department can identify and contact every individual who might have been exposed to the virus, it will not release information about these locations, it says. But the agency adds that if it can't reach everyone, it will make that geographic information public.

Public Health issued a health alert to medical providers in L.A. County Friday, informing them of the measles outbreak and urging them to consider the possibility that patients with an acute rash and fever may have the disease.

While the department says there have been sporadic cases in the past 20 months, this is the first measles outbreak in L.A. County since the one that began at the Disney theme parks in Dec. 2014. By the time that outbreak was contained in April 2015, the California Department of Public Health had confirmed 136 cases statewide. 

At least 57 of the people who got measles in that outbreak were unvaccinated, according to the California Department of Public Health. L.A. County Public Health said Thursday that the first seven cases in the current oubreak were all unvaccinated. It has not yet said whether either of the two new confirmed cases involved unvaccinated individuals.

People with measles typically develop a fever, runny nose, cough and a rash all over the body. In some people, measles can cause pneumonia, encephalitis or death.

The highly contagious disease can live for up to two hours on a surface or in the air in an area where an infected person coughed or sneezed.

If an individual has the disease, 90 percent of the people close to that person who are not immune will also become infected, according to health experts. A person can develop measles up to 21 days after being exposed to someone else who has the disease.

Infected people are usually contagious from four days before they develop a telltale rash to four days afterward.

Public Health has more information on measles here.