New York state health officials have found indications of additional cases of polio virus in wastewater samples from two different counties, leading them to warn that hundreds of people may be infected with the potentially serious virus.Just two weeks ago, the New York Health Department reported the nation's first case of polio in almost a decade, in Rockland County, north of New York City. Officials said that case occurred in a previously healthy young adult who was unvaccinated and developed paralysis in their legs. Since then, three positive wastewater samples from Rockland County and four from neighboring Orange County were discovered and genetically linked to the first case, the health department said in a press release on Thursday, suggesting that the polio virus is being spread within local communities. The newest samples were taken from two locations in Orange County in June and July and one location in Rockland County in July."Based on earlier polio outbreaks, New Yorkers should know that for every one case of paralytic polio observed, there may be hundreds of other people infected," State Health Commissioner Dr. Mary T. Bassett said. "Coupled with the latest wastewater findings, the Department is treating the single case of polio as just the tip of the iceberg of much greater potential spread. As we learn more, what we do know is clear: the danger of polio is present in New York today." The health department reiterated that it is still investigating the virus' origin, and said that it is not yet clear whether the infected person in Rockland County was linked to the other cases.Polio is "a serious and life-threatening disease," the state health department said. It is highly contagious and can be spread by people who aren't yet symptomatic. Symptoms usually appear within 30 days of infection, and can be mild or flu-like. Some people who are infected may become paralyzed or die.Before the polio vaccine was introduced in the 1950s, thousands of Americans died in polio outbreaks and tens of thousands, many of them children, were left with paralysis. After a successful vaccination campaign, polio was officially declared eradicated in the U.S. in 1979. Unvaccinated New Yorkers are encouraged to get immunized right away, the health department said. Unvaccinated people who live, work or spend time in Rockland County, Orange County and the greater New York metropolitan area are at the greatest risk.Most school-aged children have received the polio vaccine, which is a four-dose course, started between 6 weeks and 2 months of age and followed by one shot at 4 months, one at 6 to 12 months, and one between the ages of 4 and 6. According to the health department, about 60% of children in Rockland County have received three polio shots before their second birthday, as have about 59% in Orange County — both below the 79% statewide figure. According to the CDC's most recent childhood vaccination data, about 93% of 2-year-olds in the U.S. had received at least three doses of polio vaccine.Meanwhile, adults who are not vaccinated would receive a three-dose immunization, and those who are vaccinated but at high risk can receive a lifetime booster shot, according to the health department.The vaccine is 99% effective in children who receive the full four-dose regime, health officials said."It is concerning that polio, a disease that has been largely eradicated through vaccination, is now circulating in our community, especially given the low rates of vaccination for this debilitating disease in certain areas of our County," Orange County Health Commissioner Dr. Irina Gelman said. "I urge all unvaccinated Orange County residents to get vaccinated as soon as medically feasible."Rockland County Department of Health Commissioner Dr. Patricia Schnabel Ruppert issued a similar statement, calling on people who are not vaccinated to get the shots "immediately."Polio has rarely appeared in the U.S. since it was declared eradicated over 40 years ago. The last reported case was brought by a traveler in 2013, according to The Associated Press.