Those annoying yellow jackets that show up at your barbecue or picnic uninvited could bring you far more than aggravation: Their sting can be fatal for the thousands who are allergic but don't know it. September and October are the prime time for this danger, because yellow jackets are out in greater numbers then, looking for food and smells and sights they like. Those include some of the same things people are attracted to, from sandwiches and fruit and soft drinks to perfume and brightly colored clothing. Yellow jackets also like hair spray and garbage cans. A type of wasp named for their yellow markings, yellow jackets can sting people painfully and repeatedly. ``Probably about 40 or 50 people die a year from insects stings'' in the United States, mostly from yellow jackets, said Dr. Catherine Monteleone, an allergist and immunologist at Robert Wood Johnson Medical School in New Brunswick. Deaths can occur when someone allergic to yellow jacket venom gets stung, triggering an overwhelming systemic reaction, which requires immediate medical care. The problem is, such victims usually have no warning. ``Until you have a reaction, we don't know that you're allergic,'' Monteleone said. She said somewhere between 0.4 percent and 3 percent of the U.S. population is allergic to yellow jacket stings, according to some studies based on reports of patients who sought treatment from doctors or at hospitals after being stung. It's hard to have exact figures, she said, because most patients don't know enough about wasps, bees and hornets to be sure what stung them. Yellow jackets, though, can be especially dangerous because they can attack unprovoked and because they live in nests in the soil, rather than in bushes or under eaves like their other stinging relatives. ``When you're mowing the lawn or gardening, you can disturb them (yellow jackets) very easily,'' Monteleone said. For most people, a sting brings localized pain, redness and mild swelling. In someone allergic, it can start a cascade of symptoms within 15 minutes, starting with redness, hives and swelling around the lips or eyes. That can be followed by shortness of breath, tightness in the chest and throat, abdominal cramping, diarrhea, nausea, vomiting and, in women, pelvic cramps. As the person's blood pressure falls, lightheadedness, loss of consciousness and shock can follow. Immunotherapy, or a long series of allergy shots to gradually desensitize the body, is the best way to prevent such an outcome, according to Dr. Leonard Bielory, director of the Asthma and Allergy Research Center at University of Medicine and Dentistry of New Jersey in Newark. Temporary solutions for those who know they are allergic include keeping on hand epinephrine kits, which people can inject into the skin themselves, or nonprescription Benadryl, Monteleone said. Those drugs can relax constricted breathing passages and slow the allergic reaction, allowing more time to get professional medical help. Better yet, experts say, take steps to avoid being stung: Don't use scented soaps or deodorants, perfume or hair spray. Don't wear clothes with bright colors during the fall. When serving food outdoors, cover food and beverages. Avoid trash containers, which attract the pests, and be careful when drinking from open cans or bottles, because yellow jackets can be lurking inside and sting your lip or mouth.