A child with measles, a<unk> -preventable disease There is overwhelming scientific consensus that<unk> s are a very safe and effective way to fight and eradicate infectious diseases. The immune system recognizes<unk>  agents as foreign, destroys them, and "remembers" them. When the virulent version of an agent is encountered, the body recognizes the protein coat on the virus, and thus is prepared to respond, by first neutralizing the target agent before it can enter cells, and secondly by recognizing and destroying infected cells before that agent can multiply to vast numbers. Limitations to their effectiveness, nevertheless, exist. Sometimes, protection fails for<unk> -related reasons such as failures in<unk>  attenuation,<unk> tion regimens or administration. Failure may also occur for host-related reasons if the host's immune system does not respond adequately or at all. Host-related lack of response occurs in an estimated 2-10% of individuals, due to factors including genetics, immune status, age, health and nutritional status. One type of primary immunodeficiency disorder resulting in genetic failure is X-linked agammaglobulinemia, in which the absence of an enzyme essential for B cell development prevents the host's immune system from generating antibodies to a pathogen. Host–pathogen interactions and responses to infection are dynamic processes involving multiple pathways in the immune system. A host does not develop antibodies instantaneously: while the body's innate immunity may be activated in as little as twelve hours, adaptive immunity can take 1-2 weeks to fully develop. During that time, the host can still become infected. Once antibodies are produced, they may promote immunity in any of several ways, depending on the class of antibodies involved. Their success in clearing or inactivating a pathogen will depend on the amount of antibodies produced and on the extent to which those antibodies are effective at countering the strain of the pathogen involved, since different strains may be differently susceptible to a given immune reaction. In some cases<unk> s may result in partial immune protection (in which immunity is less than 100% effective but still reduces risk of infection) or in temporary immune protection (in which immunity wanes over time) rather than full or permanent immunity. They can still raise the reinfection threshold for the population as a whole and make a substantial impact. They can also mitigate the severity of infection, resulting in a lower mortality rate, lower morbidity, faster recovery from illness, and a wide range of other effects. Those who are older often display less of a response than those who are younger, a pattern known as Immunosenescence. Adjuvants commonly are used to boost immune response, particularly for older people whose immune response to a simple<unk>  may have weakened. The efficacy or performance of the<unk>  is dependent on several factors: * the disease itself (for some diseases<unk> tion performs better than for others) * the strain of<unk>  (some<unk> s are specific to, or at least most effective against, particular strains of the disease) * whether the<unk> tion schedule has been properly observed. * idiosyncratic response to<unk> tion; some individuals are "non-responders" to certain<unk> s, meaning that they do not generate antibodies even after being<unk> ted correctly. * assorted factors such as ethnicity, age, or genetic predisposition. If a<unk> ted individual does develop the disease<unk> ted against (breakthrough infection), the disease is likely to be less virulent than in unvaccinated victims. Important considerations in an effective<unk> tion program: # careful modeling to anticipate the effect that an immunization campaign will have on the epidemiology of the disease in the medium to long term # ongoing surveillance for the relevant disease following introduction of a new<unk>  # maintenance of high immunization rates, even when a disease has become rare In 1958, there were 763,094 cases of measles in the United States; 552 deaths resulted. After the introduction of new<unk> s, the number of cases dropped to fewer than 150 per year (median of 56). In early 2008, there were 64 suspected cases of measles. Fifty-four of those infections were associated with importation from another country, although only thirteen percent were actually acquired outside the United States; 63 of the 64 individuals either had never been<unk> ted against measles or were uncertain whether they had been<unk> ted.<unk> s led to the eradication of smallpox, one of the most contagious and deadly diseases in humans. Other diseases such as rubella, polio, measles, mumps, chickenpox, and typhoid are nowhere near as common as they were a hundred years ago thanks to widespread<unk> tion programs. As long as the vast majority of people are<unk> ted, it is much more difficult for an outbreak of disease to occur, let alone spread. This effect is called herd immunity. Polio, which is transmitted only among humans, is targeted by an extensive eradication campaign that has seen endemic polio restricted to only parts of three countries (Afghanistan, Nigeria, and Pakistan). However, the difficulty of reaching all children, cultural misunderstandings, and disinformation have caused the anticipated eradication date to be missed several times.<unk> s also help prevent the development of antibiotic resistance. For example, by greatly reducing the incidence of pneumonia caused by Streptococcus pneumoniae,<unk>  programs have greatly reduced the prevalence of infections resistant to penicillin or other first-line antibiotics. The measles<unk>  is estimated to prevent a million deaths every year. Adverse effects<unk> tions given to children, adolescents, or adults are generally safe. Adverse effects, if any, are generally mild. The rate of side effects depends on the<unk>  in question. Some common side effects include fever, pain around the injection site, and muscle aches. Additionally, some individuals may be allergic to ingredients in the<unk> . MMR<unk>  is rarely associated with febrile seizures. Host-<unk> e")-related determinants that render a person susceptible to infection, such as genetics, health status (underlying disease, nutrition, pregnancy, sensitivities or allergies), immune competence, age, and economic impact or cultural environment can be primary or secondary factors affecting the severity of infection and response to a<unk> . Elderly (above age 60), allergen-hypersensitive, and obese people have susceptibility to compromised immunogenicity, which prevents or inhibits<unk>  effectiveness, possibly requiring separate<unk>  technologies for these specific populations or repetitive booster<unk> tions to limit virus transmission. Severe side effects are extremely rare. Varicella<unk>  is rarely associated with complications in immunodeficient individuals, and rotavirus<unk> s are moderately associated with intussusception. At least 19 countries have no-fault compensation programs to provide compensation for those suffering severe adverse effects of<unk> tion. The United States’ program is known as the National Childhood<unk>  Injury Act, and the United Kingdom employs the<unk>  Damage Payment.
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