Why does disease occur

An individual factor that contributes to cause disease is shown as a piece of a pie. After all the pieces of a pie fall into place, the pie is complete — and disease occurs. The individual factors are called component causes.

The complete pie, which might be considered a causal pathway, is called a sufficient cause. A disease may have more than one sufficient cause, with each sufficient cause being composed of several component causes that may or may not overlap. A component that appears in every pie or pathway is called a necessary cause , because without it, disease does not occur.

Note in Figure 1. Source: Rothman KJ. Am J Epidemiol ;— The component causes may include intrinsic host factors as well as the agent and the environmental factors of the agent-host-environment triad. A single component cause is rarely a sufficient cause by itself.

For example, even exposure to a highly infectious agent such as measles virus does not invariably result in measles disease. Host susceptibility and other host factors also may play a role.

At the other extreme, an agent that is usually harmless in healthy persons may cause devastating disease under different conditions. Pneumocystis carinii is an organism that harmlessly colonizes the respiratory tract of some healthy persons, but can cause potentially lethal pneumonia in persons whose immune systems have been weakened by human immunodeficiency virus HIV.

Presence of Pneumocystis carinii organisms is therefore a necessary but not sufficient cause of pneumocystis pneumonia. In Figure 1. As the model indicates, a particular disease may result from a variety of different sufficient causes or pathways. For example, lung cancer may result from a sufficient cause that includes smoking as a component cause. Smoking is not a sufficient cause by itself, however, because not all smokers develop lung cancer.

Neither is smoking a necessary cause, because a small fraction of lung cancer victims have never smoked. Sufficient Cause I includes both smoking B and asbestos C. But because lung cancer can develop in persons who have never been exposed to either smoking or asbestos, a proper model for lung cancer would have to show at least one more Sufficient Cause Pie that does not include either component B or component C.

Note that public health action does not depend on the identification of every component cause. Disease prevention can be accomplished by blocking any single component of a sufficient cause, at least through that pathway.

For example, elimination of smoking component B would prevent lung cancer from sufficient causes I and II, although some lung cancer would still occur through sufficient cause III. Check your answer. Anthrax is an acute infectious disease that usually occurs in animals such as livestock, but can also affect humans.

Human anthrax comes in three forms, depending on the route of infection: cutaneous skin anthrax, inhalation anthrax, and intestinal anthrax.

Symptoms usually occur within 7 days after exposure. Skin infection begins as a raised itchy bump that resembles an insect bite but within 1—2 days develops into a vesicle and then a painless ulcer, usually 1—3 cm in diameter, with a characteristic black necrotic dying area in the center.

Lymph glands in the adjacent area may swell. Deaths are rare with appropriate antimicrobial therapy. Inhalation: Initial symptoms are like cold or flu symptoms and can include a sore throat, mild fever, and muscle aches. After several days, the symptoms may progress to cough, chest discomfort, severe breathing problems and shock. Inhalation anthrax is often fatal.

Intestinal: Initial signs of nausea, loss of appetite, vomiting, and fever are followed by abdominal pain, vomiting of blood, and severe diarrhea. While most human cases of anthrax result from contact with infected animals or contaminated animal products, anthrax also can be used as a biologic weapon. In , dozens of residents of Sverdlovsk in the former Soviet Union are thought to have died of inhalation anthrax after an unintentional release of an aerosol from a biologic weapons facility.

This level is not necessarily the desired level, which may in fact be zero, but rather is the observed level.

In the absence of intervention and assuming that the level is not high enough to deplete the pool of susceptible persons, the disease may continue to occur at this level indefinitely. Thus, the baseline level is often regarded as the expected level of the disease. While some diseases are so rare in a given population that a single case warrants an epidemiologic investigation e.

Sporadic refers to a disease that occurs infrequently and irregularly. Hyperendemic refers to persistent, high levels of disease occurrence. Occasionally, the amount of disease in a community rises above the expected level.

Epidemic refers to an increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area. Outbreak carries the same definition of epidemic, but is often used for a more limited geographic area. Cluster refers to an aggregation of cases grouped in place and time that are suspected to be greater than the number expected, even though the expected number may not be known.

Pandemic refers to an epidemic that has spread over several countries or continents, usually affecting a large number of people. Epidemics occur when an agent and susceptible hosts are present in adequate numbers, and the agent can be effectively conveyed from a source to the susceptible hosts.

More specifically, an epidemic may result from:. The previous description of epidemics presumes only infectious agents, but non-infectious diseases such as diabetes and obesity exist in epidemic proportion in the U. Check your answer. A common-source outbreak is one in which a group of persons are all exposed to an infectious agent or a toxin from the same source. If the group is exposed over a relatively brief period, so that everyone who becomes ill does so within one incubation period, then the common-source outbreak is further classified as a point-source outbreak.

The epidemic of leukemia cases in Hiroshima following the atomic bomb blast and the epidemic of hepatitis A among patrons of the Pennsylvania restaurant who ate green onions each had a point source of exposure. Figure 1. Image Description. Source: Centers for Disease Control and Prevention. Unpublished data; In some common-source outbreaks, case-patients may have been exposed over a period of days, weeks, or longer. In a continuous common-source outbreak , the range of exposures and range of incubation periods tend to flatten and widen the peaks of the epidemic curve Figure 1.

The epidemic curve of an intermittent common-source outbreak often has a pattern reflecting the intermittent nature of the exposure. A propagated outbreak results from transmission from one person to another. It also has an envelope with crown-like spikes on its surface. Seven coronaviruses can affect humans, but each one can change or mutate, producing many variants.

Learn more about coronaviruses here. Just as there are friendly bacteria in the intestines that are essential to gut health , humans may also carry friendly viruses that help protect against dangerous bacteria, including Escherichia coli.

Viruses do not leave fossil remains, so they are difficult to trace through time. Scientists use molecular techniques to compare the DNA and RNA of viruses and find out more about where they come from. Three competing theories try to explain the origin of viruses. In reality, viruses may have evolved in any of these ways. The regressive, or reduction, hypothesis suggests that viruses started as independent biological entities that became parasites.

Over time, they shed genes that did not help them parasitize, and became entirely dependent on the cells they inhabit. In this way, they gained the ability to become independent and move between cells.

The virus-first hypothesis suggests that viruses evolved from complex molecules of nucleic acid and proteins either before or at the same time as the first cells on Earth appeared, billions of years ago. When a viral disease emerges, it is not always clear where it comes from. A virus exists only to reproduce. When it reproduces, particles spread to new cells and new hosts. The features of a virus affect its ability to spread.

Some viruses can remain active on an object for some time. If a person with the virus on their hands touches an item, the next person can pick up that virus by touching the same object.

The object is known as a fomite. Viruses often change over time. Some of these changes are very small and do not cause concern, but others can be more significant. Significant changes could make a virus more transmissible, as has been the case with the B. They may also help the virus evade the immune system or existing treatments.

For example, doctors use several drugs in combination to treat HIV so that it is harder for the virus to develop resistance to treatment. Influenza viruses can also do so-called antigenic shift. This can happen if a host cell has become infected with two different types of influenza virus. For instance, pigs can often serve as a mixing vessel for avian and human influenza viruses.

Some viruses, such as HPV, can lead to cancer. The full impact of a virus can take time to appear, and sometimes there may be a secondary effect. For example, the herpes zoster virus can cause chickenpox. The person recovers, but the virus may stay in the body. Years later, it may cause shingles in the same individual. Coronaviruses are a large family of viruses and include viruses that cause the common cold.

However, it has changed many times since scientists first identified it in China. By September , scientists had logged over 12, mutations, and the development continues. Some variants are more transmissible and more likely to cause severe illness than others. The main concern with new variants is the unpredictability of their impact. The main symptoms of COVID are dry cough , fatigue , and fever, but there are many possible symptoms. Anyone who has symptoms should seek a test.