Microbiology

Infectious diseases are those caused by microorganisms. In order to relate a particular organism to a particular disease, Koch's postulates must be fulfilled. First devised by Robert Koch in the 1870s, Koch's postulates are a series of procedures for identifying the cause of a particular disease. They are described in the first chapter of this book.

Symptoms and signs. Infectious diseases are usually characterized by changes in body function known as symptoms. Symptoms are subjective changes not always apparent to the observer. The patient may also exhibit signs, which are objective changes that can be measured. Fever and a skin rash are examples of signs. When a specific group of symptoms or signs accompanies a disease, the group is called asyndrome.

Transmission and incidence. Infectious diseases may be classified according to theirtransmissibility. A disease that spreads from one host to another is a communicable disease. Those communicable diseases transmitted with particular ease are said to becontagious. Diseases not spread between hosts are noncommunicable.Staphylococcal food poisoning is an example.

The incidence of a disease refers to the percent of a population that contracts it over a particular period. The prevalence of a disease, by contrast, is the percentage of a population having the disease at a particular time.

When a disease occurs only occasionally, it is called a sporadic disease. A disease present in a population at all times is an endemic disease. A disease that breaks out in a population in a short period is an epidemic disease, and an epidemic disease occurring throughout the world is a pandemic disease.

Types of disease. Diseases can be defined in terms of their severity and duration. Anacute disease occurs rapidly and lasts a short time, while a chronic disease develops slowly and lasts a long time. Influenza is an acute disease, while tuberculosis is a chronic disease. A subacute disease is a disease that has vague symptoms and lasts a relatively long time. A latent disease remains inactive in a host for a time and then becomes active.

Infections can be described as local infections if they are restricted to a small area of the body and systemic infections if they spread throughout the body systems. The presence of multiplying microorganisms in the blood is septicemia. Toxins present in the blood constitute toxemia.

Infectious diseases may also be described as primary diseases or secondary diseases. Aprimary disease is the first illness that occurs, and a secondary disease is due to an opportunistic microorganism, often a normal resident, after the body's defenses have weakened.

Modes of disease transmission. When a disease remains in a population, a source of pathogens called a reservoir of infection exists within the population. The reservoir can be human, animal, or nonliving, such as the soil. A human reservoir who has had the disease and recovered but continues to shed infectious organisms is called acarrier. Animal diseases spread to humans are called zoonoses.

Among the principal routes of transmission of disease are contact, vectors, and vehicles. Contact can be direct or indirect. Direct transmission occurs from person to person by such things as touching, kissing, and sexual intercourse (Figure 1 ). Indirect transmission occurs when a nonliving object is intermediary between two humans. 

Figure 1

Some modes of transmission of microorganisms from the respiratory and oral tract.

A lifeless object known as a fomite is often involved in disease transmission. The object may be a towel, cup, or eating utensil. Transmission can also be effected by droplet nuclei, bits of mucus and saliva that spread between individuals.

Vectors are living things. Arthropods such as mosquitoes, flies, and ticks may carry pathogens on their body parts, in which case they are mechanical vectors. If the arthropod is infected and transmits the organism in its saliva or feces, it is a biological vector.

Vehicles are lifeless objects such as food, water, and air. Water may be contaminated by human feces, while food is often contaminated by pathogens from the soil. Air can be a vehicle for transmission for droplet nuclei in such diseases as tuberculosis and common colds.

In order for infection to be transmitted, microorganisms must leave the body through aportal of exit, which can be the intestine, mouth, or skin surface. Generally, the portal of exit is the same as the infected body part. Organisms enter the new individual through a portal of entry.

Infections acquired during a hospital stay are called nosocomial infections. These infections often occur in compromised hosts who are being treated for other conditions such as cancer, nutritional deficiency, burns, or other forms of stress.

Disease patterns. When a disease develops in an individual, a recognized set of periods can be identified. The first period is the period of incubation, the time between the entry of the parasite into the host and the appearance of symptoms.

The next period is the prodrome period. This period is accompanied by mild symptoms such as aches, fever, and early signs of disease.

Next comes the period of illness, when the disease is most acute. Signs and symptoms are most apparent, and each disease has its own characteristic appearance. The body's immune system is activated during this period, and specific defense is critical to recovery.

The final periods are the periods of decline and convalescence. The period of decline is one in which the signs and symptoms subside, and during the period of convalescence, the person returns to normal. After the disease has abated, the immune system continues to produce antimicrobial factors that will ensure long-term immunity.