How does disease occur

Viral pathogens associated with diarrheal disease include norovirus and rotavirus. Parasitic pathogens associated with diarrhea include Giardia lamblia and Cryptosporidium parvum.

Likewise, fever is indicative of many types of infection, from the common cold to the deadly Ebola hemorrhagic fever. Finally, some diseases may be asymptomatic or subclinical , meaning they do not present any noticeable signs or symptoms. For example, most individual infected with herpes simplex virus remain asymptomatic and are unaware that they have been infected.

In this section, we will introduce terminology used by the ICD and in health-care professions in general to describe and categorize various types of disease. An infectious disease is any disease caused by the direct effect of a pathogen. A pathogen may be cellular bacteria, parasites, and fungi or acellular viruses, viroids, and prions.

Some infectious diseases are also communicable , meaning they are capable of being spread from person to person through either direct or indirect mechanisms. Some infectious communicable diseases are also considered contagious diseases, meaning they are easily spread from person to person.

Not all contagious diseases are equally so; the degree to which a disease is contagious usually depends on how the pathogen is transmitted. For example, measles is a highly contagious viral disease that can be transmitted when an infected person coughs or sneezes and an uninfected person breathes in droplets containing the virus.

Gonorrhea is not as contagious as measles because transmission of the pathogen Neisseria gonorrhoeae requires close intimate contact usually sexual between an infected person and an uninfected person. Diseases that are contracted as the result of a medical procedure are known as iatrogenic disease s.

Iatrogenic diseases can occur after procedures involving wound treatments, catheterization, or surgery if the wound or surgical site becomes contaminated. Diseases acquired in hospital settings are known as nosocomial disease s. Several factors contribute to the prevalence and severity of nosocomial diseases. First, sick patients bring numerous pathogens into hospitals, and some of these pathogens can be transmitted easily via improperly sterilized medical equipment, bed sheets, call buttons, door handles, or by clinicians, nurses, or therapists who do not wash their hands before touching a patient.

Second, many hospital patients have weakened immune systems, making them more susceptible to infections. Compounding this, the prevalence of antibiotics in hospital settings can select for drug-resistant bacteria that can cause very serious infections that are difficult to treat. Certain infectious diseases are not transmitted between humans directly but can be transmitted from animals to humans. Such a disease is called zoonotic disease or zoonosis.

According to WHO, a zoonosis is a disease that occurs when a pathogen is transferred from a vertebrate animal to a human; however, sometimes the term is defined more broadly to include diseases transmitted by all animals including invertebrates. For example, rabies is a viral zoonotic disease spread from animals to humans through bites and contact with infected saliva.

Many other zoonotic diseases rely on insects or other arthropods for transmission. Examples include yellow fever transmitted through the bite of mosquitoes infected with yellow fever virus and Rocky Mountain spotted fever transmitted through the bite of ticks infected with Rickettsia rickettsii. In contrast to communicable infectious diseases, a noncommunicable infectious disease is not spread from one person to another.

One example is tetanus , caused by Clostridium tetani , a bacterium that produces endospores that can survive in the soil for many years. This disease is typically only transmitted through contact with a skin wound; it cannot be passed from an infected person to another person. Similarly, Legionnaires disease is caused by Legionella pneumophila , a bacterium that lives within amoebae in moist locations like water-cooling towers.

An individual may contract Legionnaires disease via contact with the contaminated water, but once infected, the individual cannot pass the pathogen to other individuals.

In addition to the wide variety of noncommunicable infectious diseases, noninfectious disease s those not caused by pathogens are an important cause of morbidity and mortality worldwide.

Noninfectious diseases can be caused by a wide variety factors, including genetics, the environment, or immune system dysfunction, to name a few. For example, sickle cell anemia is an inherited disease caused by a genetic mutation that can be passed from parent to offspring Figure 1.

Other types of noninfectious diseases are listed in Table 2. Figure 1. Blood smears showing two diseases of the blood. It is transmitted by mosquitoes to humans. It is not caused by a pathogen, but rather a genetic mutation. The five periods of disease sometimes referred to as stages or phases include the incubation, prodromal, illness, decline, and convalescence periods Figure 2.

The incubation period occurs in an acute disease after the initial entry of the pathogen into the host patient. It is during this time the pathogen begins multiplying in the host.

However, there are insufficient numbers of pathogen particles cells or viruses present to cause signs and symptoms of disease. Incubation periods can vary from a day or two in acute disease to months or years in chronic disease, depending upon the pathogen. Factors involved in determining the length of the incubation period are diverse, and can include strength of the pathogen, strength of the host immune defenses, site of infection, type of infection, and the size infectious dose received.

During this incubation period, the patient is unaware that a disease is beginning to develop. Figure 2. The progression of an infectious disease can be divided into five periods, which are related to the number of pathogen particles red and the severity of signs and symptoms blue.

The prodromal period occurs after the incubation period. During this phase, the pathogen continues to multiply and the host begins to experience general signs and symptoms of illness, which typically result from activation of the immune system, such as fever, pain, soreness, swelling, or inflammation.

Usually, such signs and symptoms are too general to indicate a particular disease. Following the prodromal period is the period of illness , during which the signs and symptoms of disease are most obvious and severe. The period of illness is followed by the period of decline , during which the number of pathogen particles begins to decrease, and the signs and symptoms of illness begin to decline.

Viruses make us sick by killing cells or disrupting cell function. Many bacteria make us sick in the same way that viruses do, but they also have other strategies at their disposal. Sometimes bacteria multiply so rapidly they crowd out host tissues and disrupt normal function. Sometimes they kill cells and tissues outright.

The protozoa that cause malaria , which are members of the genus Plasmodium, have complex life cycles. Sporozoites, the stage of the parasite that infects new hosts, develop in the salivary glands of Anopheles mosquitos. Cells infected with sporozoites eventually burst, releasing another cell form, merozoites, into the bloodstream.

These cells infect red blood cells and then rapidly reproduce, destroying the red blood cell hosts and releasing many new merozoites to do further damage. Most merozoites continue to reproduce in this way, but some differentiate into sexual forms gametocytes that are taken up by the female mosquito, thus completing the protozoan life cycle.

When you touch a doorknob handled by someone ill with the flu or a cold, for example, you can pick up the germs he or she left behind. If you then touch your eyes, mouth or nose before washing your hands, you may become infected. Some germs rely on insect carriers — such as mosquitoes, fleas, lice or ticks — to move from host to host.

These carriers are known as vectors. Mosquitoes can carry the malaria parasite or West Nile virus. Deer ticks may carry the bacterium that causes Lyme disease. Disease-causing germs can also infect you through contaminated food and water.

This mechanism of transmission allows germs to be spread to many people through a single source. Escherichia coli E. While anyone can catch infectious diseases, you may be more likely to get sick if your immune system isn't working properly. This may occur if:. In addition, certain other medical conditions may predispose you to infection, including implanted medical devices, malnutrition and extremes of age, among others.

Most infectious diseases have only minor complications. But some infections — such as pneumonia, AIDS and meningitis — can become life-threatening. A few types of infections have been linked to a long-term increased risk of cancer:.

In addition, some infectious diseases may become silent, only to appear again in the future — sometimes even decades later. For example, someone who's had chickenpox may develop shingles much later in life.

Prepare food safely. Keep counters and other kitchen surfaces clean when preparing meals. Cook foods to the proper temperature, using a food thermometer to check for doneness. For ground meats, that means at least F 71 C ; for poultry, F 74 C ; and for most other meats, at least F 63 C.

Also promptly refrigerate leftovers — don't let cooked foods remain at room temperature for long periods of time.

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