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An outbreak is a sudden rise in the occurrence (the number of cases) of a disease. Some outbreaks are expected each year, like influenza, hand-foot-and-mouth disease, and bronchiolitis. This term might be used when a single case of an unusual disease occurs. In 2006, a team of researchers, epidemiologists and informaticians at Children's Hospital Boston developed HealthMap, an online tool for disease outbreak monitoring and real-time surveillance of emerging public health threats.

The definition of an influenza pandemic given by the World Health Organization (WHO) states that “an influenza pandemic occurs when a new influenza virus appears against which the human population has no immunity, resulting in several simultaneous epidemics worldwide with enormous numbers of deaths and illness”.
Such a definition has been changed in 2009, in concomitance with the H1N1 influenza outbreak, and lost part of its severe meaning (see also Swine flu).
The occurrence of pandemics is rare, and are different compared to regular seasonal epidemics of influenza. The most severe influenza pandemic in recent history was the 1918 Spanish flu, which is estimated being responsible for the deaths of over 50 million people.

Swine influenza, also called pig influenza, “swine flu”, “hog flu” and “pig flu”, is an infection caused by any one of several types of swine influenza viruses, including influenza C virus and the subtypes of influenza A (H1N1, H1N2, H2N1, H3N1, H3N2, and H2N3). Swine influenza virus is common throughout pig populations worldwide. Transmission of the virus from pigs to humans is not common and does not always lead to human flu, often resulting only in the production of antibodies in the blood. If transmission does cause human flu, it is called zoonotic swine flu. People with regular exposure to pigs are at increased risk of swine flu infection. Symptoms of zoonotic swine flu in humans are similar to those of influenza and of influenza-like illness in general, namely chills, fever, sore throat, muscle pains, severe headache, coughing, weakness and general discomfort.
In April 2009, a new strain of H1N1 virus, responsible for human influenza, was originally referred to as “swine flu” because laboratory testing showed that many of the genes in the virus were very similar to influenza viruses that normally occur in pigs in North America. However, further study has shown that the 2009 H1N1 is very different from what normally circulates in North American pigs. It has two genes from flu viruses that normally circulate in pigs in Europe and Asia and others from bird (avian) genes and human flu viruses genes. Scientists call this a "quadruple reassortant" virus.
Despite its name, the H1N1 “swine flu” virus cannot be spread by eating pork or pork products and, similar to other influenza viruses, it is typically contracted by person to person transmission through respiratory droplets. Symptoms usually last 4–6 days. Unlike most strains of influenza, H1N1 does not disproportionately infect adults older than 60 years. This virus (Probably due to previous exposure to a similar H1N1 virus) was first detected in people in the United States in April 2009 and it was as a pandemic by the World Health Organization in June 2009. It spread worldwide and, as of May 30 2010, 18138 deaths have been reported. On August 10, 2010, the World Health Organization declared the H1N1 influenza pandemic over, saying worldwide flu activity had returned to typical seasonal patterns.
An intense debate arose about the declaration of the “swine flu” as a pandemic. Critics claimed the WHO had exaggerated the danger, spreading fear and confusion rather than immediate information.
A flu follow-up study done in September 2010, found that "the risk of most serious complications was not elevated in adults or children". A 2011 study estimated that the 2009 H1N1 global infection rate was 11% to 21% lower than what was previously expected while another one, published in 2012, showed that as many as 579,000 people could have been killed by the disease, as only those fatalities confirmed by laboratory testing were included in the original number, and meant that many of those without access to health facilities went uncounted. The majority of these deaths occurred in Africa and South East Asia.

Preparedness refers to the state of being prepared for specific or unpredictable events or situations. Specifically, public health preparedness is fundamental to avoid and mitigate negative outcomes due to infectious disease outbreaks. It is a major phase of emergency management which relies on research, estimation, planning, resourcing, education, practicing and rehearsing. Preparedness of the legal environment in which public health authorities will operate is also crucial. Public law legal preparedness refers to the concept of ensuring an adequate level of legal preparedness for public health emergency situations. This is because if the correct legal framework is not in place at the time the emergency occurs it may not be possible to implement the required response. The International Health Regulations obliges states to be prepared for an epidemic situation and also obliges respect for human rights in such planning. States therefore have an obligation to consider human rights implications during planning for a potential epidemic situation under their obligations under the IHR. This includes human rights within the concept of public law preparedness that has been a hallmark of states’ disaster preparation since September 2001.