City doctors have been alerted to be on the lookout for measles following the confirmation of two cases in Brooklyn.

The two cases occurred in members of a single family, according to health officials, who issued the warning to doctors last week. Measles, the frequency of which dropped off following the introduction of a vaccine in the 1960s, is characterized by fever and an itchy rash that starts on the face and moves down the body.

Health officials said they issued the alert as a “precautionary, proactive” measure. “There’s no reason for alarm,” the health department’s director of epidemiology and surveillance, Dr. Christopher Zimmerman, said. “If you are vaccinated against measles, you will not get measles.”

Regardless, New York parents have taken their concerns to the blogosphere, which has been buzzing with rumors of an outbreak. On one parenting Web site, UrbanBaby.com, a mother said she was “getting paranoid about the measles.” Another wrote: “I’m not panicked, but measles?”

The city’s health department investigates all measles cases. Last year, officials confirmed five cases out of 36 suspected infections. The last outbreak of measles was in 2000, when 10 cases were confirmed.

In large part, the incidence of measles in America has declined in recent decades. Currently, there are outbreaks in California, Belgium, Britain, Israel, and some developing countries. Indigenous cases are rare in America. “Young doctors never see a case unless they work out of the country,” an epidemiologist at SUNY Downstate Medical Center, Dr. Michael Augenbraun, said.

In New York City, the measles vaccine is required for admission to the city’s day care centers, schools, and colleges.

While health officials have said there is no cause for alarm, some doctors said the alert heightened their sensitivity to measles.

“I’m on the looking out now,” a pediatrician who practices in Park Slope, Dr. Philippa Gordon, said. Earlier this week, Dr. Gordon created a stir among parents after she posted the health department alert on a parenting Web site, ParkSlopeParents.com. “I actually felt bad because I think I triggered a little bit of panic,” she said.

An attending physician at Methodist Hospital’s pediatric emergency department, Dr. Christopher Kelly, said he had not witnessed anxiety among parents, although he would be watching closely for measles cases.

“This time of year, we see a lot of kids with fever, cough, runny nose, which are the primary symptoms of measles, so you always think about it, but most of them don’t have the characteristic rash,” he said. “Now that we’ve seen it in the area, you keep it in the back of your mind.”