Epidemiology J Endemic, epidemic or pandemic? Disease prevention Birth of a pandemic

Endemic, epidemic or pandemic? A disease is described as endemic when it is constantly present in a particular group of people. The common cold is endemic in New Zealand schools because on any day it is extremely likely that at least one student will have a cold. An epidemic is a widespread outbreak of an infectious disease, with a significant increase in the number of cases in a particular area. 10 or 15 cases of a disease not normally seen at all would be classified as an epidemic, while a relatively common disease might not reach epidemic status unless there are more than 500 cases at one time. The area covered by the epidemic might be as small as a single school or rest home, a suburb, a city or even a country.

Diseases are classified as pandemics when they reach epidemic proportions across a very wide area — such as a group of countries, a continent, or the whole world. The 1918 influenza pandemic (Spanish Flu) was so widespread that it infected one person in three across the entire world. About 15% of those infected died. So many people fell ill in 1918 that temporary hospitals were created such as this one in a town hall.

Endemic, epidemic or pandemic? A disease is endemic if it is constantly present in a given community: example, the common cold. An epidemic occurs when there is a significant increase in cases in an area: example, an epidemic of whooping cough in Wellington. A pandemic is a very wide-spread epidemic, covering several countries to the whole world.

Disease prevention Vaccination provides the best protection from disease. Those fully vaccinated against a disease are highly unlikely to catch it, and when most of the population is immune it is difficult for the disease to spread. This protects vulnerable people such as the very young or people whose immune system is weak (such as cancer patients or people who have had organ transplants). Smallpox was a disease that used to kill millions, but through vaccination campaigns around the world this disease has now been eradicated. The World Health Organisation is currently using vaccination to eradicate polio, a disease that causes paralysis in many children.

Where there is no vaccine, the epidemic must be stopped by strict quarantine of both infected patients and those exposed but not yet sick. Frequent hand washing for both patients and those near them will reduce the chance of disease transmission. Touch surfaces, especially door knobs, light switches and taps, should be wiped frequently with a dilute solution of chlorine bleach. Covering coughs and sneezes reduces transmission of many viruses.

Disease prevention Vaccination is the best way to prevent and halt epidemics. People who have been vaccinated are highly unlikely to develop the disease, and when a large proportion of the population are immune, the disease is unable to spread. Disease transmission can be reduced by quarantining of sick or exposed people. Frequent hand washing and wiping touch surfaces such as door handles, taps and light switches with chlorine bleach solution also reduces the risk of disease transmission. 6J 1 Preventing disease 6J 2 Nasty norovirus 6J 3 Outbreak!

Birth of a pandemic For a pandemic to occur, a large number of people must get sick with a disease across a number of countries. Since they got sick, those people must not have had immunity to that disease. Such widespread lack of immunity is most likely if the pathogen is significantly different from previous strains. A new strain of a virus – such as the H1N1 2009 swine flu – develops when an individual becomes infected with two different strains of a virus at once. In the case of swine flu, a pig was infected with both swine influenza and human influenza. Reassortment of the viral genetic material took place to produce a new virus with a mixture of genetic material from each parent virus.

Reassortment of viral genes produces a change known as antigenic shift Reassortment of viral genes produces a change known as antigenic shift. The new virus is sufficiently different from its parents that antibodies to either one of the parent viruses will not respond to the new virus. Thus most individuals will have no immunity and a pandemic results. Most viruses do not change unless reassortment takes place. Normally a single illness, or course of vaccination, provides life-long immunity. A few viruses, including influenza and norovirus (which causes diarrhoea and vomiting), undergo antigenic drift as small changes are made to surface proteins which give the virus a better chance of evading antibodies. That’s why flu vaccines are updated each year.

Birth of a pandemic Pandemics occur when a new virus develops. Because no-one has immunity to the new virus, large numbers of people get sick. New viruses develop when a single host becomes infected with two different strains of the same virus – such as a pig being infected with a version of swine flu and a human flu virus. Reassortment takes place as genetic material from the two strains is combined to make a new virus. This is also called antigenic shift. A few viruses such as influenza also undergo frequent minor changes in surface antigens, called antigenic drift. These changes can allow the virus to sneak past antibodies developed for an older version of the virus. 6J 4 Inside influenza 6J 5 The HIV/AIDS pandemic

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