Identifying Pathogens and Transmission Vectors

Before researchers can develop an effective vaccine, they must isolate the agent causing the disease. A series of steps known as Koch’s postulates has guided scientists through the process of isolating a pathogen and demonstrating that it causes the disease in question.

Because bacteria are larger than viruses and can be seen through a microscope, most of the early identifications of disease agents involved bacteria. After 1900, scientists began to isolate and identify viral agents as well, though viruses would not be imaged until mid-century.

Last update 7 February 2019

Timeline Entry: 1865

Disease of Silkworms

Pasteur accepted a task to investigate a disease of the silkworm that was ravaging France’s silk industry.

Pasteur, never having handled a silkworm before, traveled to the south of France to investigate. Soon after, he had isolated the microorganism causing the disease. This work contributed to Pasteur’s growing interest in infectious disease.

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Timeline Entry: 08/14/1881

Carlos Finlay Identifies a Suspect

Carlos Finlay (1833-1915) presented the paper “The Mosquito Hypothetically Considered as the Transmitting Agent of Yellow Fever” to Havana’s Academy of Sciences—the first to correctly identify mosquitoes as the ultimate source of the disease. Finlay’s theory, however, was initially ridiculed. It was accepted only when U.S. Army scientists working under Walter Reed (1851-1902) demonstrated that it was correct—two decades later.

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Timeline Entry: 1883

Bacterium Identified

Edwin Klebs (1834-1913), a Swiss-German pathologist, identified and described the bacterium that causes diphtheria. It was known at first as the Klebs-Loeffler bacterium.

The bacterium’s club-shaped appearance helped Klebs differentiate it from other microbes. Later, the bacterium became known as Microsporon diphtheriticum, Bacillus diphtheriae, Mycobacterium diphtheriae, and, as it is called now, Corynebacterium diphtheriae.

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Timeline Entry: 1884

Loeffler's Contributions

Friedrich Loeffler (1852-1915), a German bacteriologist, was the first to cultivate Corynebacterium diphtheriae.

Loeffler used a set of rules we now know as Koch's postulates to confirm that Corynebacterium diphtheriae was the agent that caused diphtheria. Later in 1884, Loeffler showed that C. diphtheriae produces a toxin. This was the first description of a bacterial exotoxin. (An exotoxin is a toxin excreted into the surrounding environment by a microbe.)

When C. diphtheriae colonizes the upper respiratory tract, the bacteria secrete a toxin that injures and then destroys cells. Waste products and proteins form a thick gray substance called a pseudomembrane over the pharynx. The pseudomembrane sticks to tissues and may obstruct breathing. Meanwhile, the toxin may travel to the heart, muscle, kidneys, liver, and other areas.

Koch’s Postulates

Robert Koch and Friedrich Loeffler’s steps formed the basis for identifying a disease-causing agent.

  1. The microbe is present in every case of the disease.
  2. The microbe can be taken from the host and grown independently.
  3. The disease can be produced by introducing a pure culture of the microbe into a healthy host.*
  4. The microbe can be isolated and identified from the host infected in Step 3.

*One exception to Step 3 is that some individuals may be infected with a disease-causing microbe and not show signs of the disease. These are known as asymptomatic carriers.

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Timeline Entry: 1897

Bacterium Mistakenly Blamed for Yellow Fever

Italian bacteriologist Giuseppe Sanarelli (1865-1940), a prominent researcher in his field, announced that a bacillus bacterium was the cause of yellow fever. Initial investigations seemed to bear this out; however, United States Army Surgeon Walter Reed eventually demonstrated that Sanarelli’s Bacillus icteroides was a secondary invader, and was not the cause of yellow fever.

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Timeline Entry: 1900

U.S. Army Researchers Discover the Cause of Yellow Fever

Spurred by the massive yellow fever-related casualties in the Spanish-American War, members of the U.S. Army Yellow Fever Commission, headed by Walter Reed, traveled to Cuba to study the disease. Commission member Jesse Lazear (1866-1900) met Henry Rose Carter, a surgeon who had studied the incubation period of yellow fever. Carter gave Lazear a draft of a paper he’d authored defining the incubation period for the disease as a range of 10 to 17 days. Lazear reportedly stated that Carter’s dates, should they prove to be correct, suggested a living host for yellow fever. Determined to uncover the source of the illness, the Commission members visited Carlos Finlay and decided to test his theory of mosquito transmission.

Jesse Lazear hatched Finlay’s mosquito eggs and let the mosquitoes feed on patients infected with yellow fever at a Havana hospital. The mosquitoes were then allowed to feed on study volunteers over a period of two weeks—yet no infections resulted. Two days later, however, Lazear once again allowed the mosquitoes to feed. This time, both of the men who were bitten fell ill. These experiments validated the theory Finlay had presented two decades earlier: mosquitoes (specifically, the Aedes aegypti variety) were the transmission vector of yellow fever. The researchers went on to rule out a bacterium as the disease agent. They determined that an infectious particle too small to be filtered with a standard bacterial filter was the source of the disease: the first human virus ever discovered. (German scientists had identified the virus that caused foot-and-mouth disease in animals in 1898.)

The two men Lazear exposed to yellow fever via the experiment’s mosquitoes recovered. Lazear himself, however, was not so lucky. It is likely that he allowed himself to be bitten as part of the experiment. Lazear contracted yellow fever and died in September 1900, at age 34.

The Army experiments, however, continued, with Reed naming newly designed facilities “Camp Lazear.” The group proceeded to determine that the mosquitoes could transmit the disease only after a certain period of time had passed since they had fed on another human infected with it (in the range of 12-20 days) and that a victim bitten by an infected mosquito would typically fall ill within six days.

William Crawford Gorgas (1854-1920), a Colonel in the U.S. Army Medical Corps, later described the details of the discovery:

They… found out that [a man], before he had been bitten by the yellow-fever mosquito, could sleep in the bed in which a patient had died of yellow fever, could be covered with a black vomit from a yellow-fever patient, or be exposed to the emanations from yellow fever in any other way, and as long as he was kept safe from the bite of the mosquito he would not have yellow fever; but this same man, after all this exposure, if afterwards bitten by an infected mosquito, would very certainly catch the disease.

…They had a little frame building built in this camp furnished with jars and the necessary simple material for breeding mosquitoes… Eggs of this particular species of mosquito were obtained and hatched in one of the jars. A female mosquito was taken from the booth thus hatched. The male mosquito will not bite… The female mosquito selected was put into a small glass tube, stoppered with a little cotton, so that she could get air but not escape, taken to Habana, placed on the hand of a patient in the first three days of an attack of yellow fever, and allowed to fill herself with blood. She was then brought back to her former home, placed in a large glass jar, and allowed to digest the blood she had obtained.

…So confident were the men in charge of the mosquitoes that I have known them to put their hands in the jars and let the mosquitoes feed upon them, up to the fifth or sixth day after the mosquito had bitten a yellow-fever patient. The mosquito, you recollect, can not convey the disease till from 12 to 20 days have passed from the time of her biting the yellow-fever case from which she becomes infected.

On the other hand, I once saw a party of 12 or 15 doctors in the mosquito room one day, when the mosquito-bar covering of the jar accidentally came off and the insects escaped into the room. These doctors had come from other countries to investigate the subject, and were not then convinced that the mosquito carried yellow fever. Still, they did not care to put the matter to a practical test in their own persons, and got out of the room so rapidly that the wire-screen door was broken down during their exit. It happened that the mosquitoes in this jar had never bitten a yellow-fever patient and were not infected.

-- Colonel William C. Gorgas, Medical Corps, United States Army, A few general directions with regard to destroying mosquitoes, particularly the yellow-fever mosquito

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Timeline Entry: 1908

Poliovirus Identified

In Vienna, Karl Landsteiner, MD (1868-1943), and Erwin Popper, MD (1879-1955), announced that the infectious agent in polio was a virus.

Popper and Landsteiner deduced the viral nature of polio by carefully filtering preparations of spinal cord fluid from a person who had died of polio. The filters were known to trap bacteria. When Popper and Landsteiner injected the filtered preparations into monkeys, the monkeys developed polio. The researchers then concluded that an infectious particle smaller than bacteria caused the disease.

Poliovirus itself would not be visible to researchers until the 1950s, when the electron microscope was available.

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Timeline Entry: 1953

Adenoviruses Identified

While working at the Walter Reed Army Institute of Research (WRAIR), Maurice Hilleman flew to Missouri to investigate an influenza outbreak in Army troops. When he returned to his lab, however, he discovered that he had isolated an entirely new type of virus. Today that family of virus is known as the adenoviruses. They cause up to 10% of all upper respiratory infections in children and affect adults as well.

Adenoviruses were a major cause of acute respiratory distress syndrome among military trainees during their initial basic combat training. Building on Hilleman's work, an inactivated adenovirus vaccine of marginal value was developed in 1956. A more efficacious live virus vaccine was developed by other scientists in the 1970s and widely used in the military from decades.

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Timeline Entry: 1954

Thomas Peebles Isolates the Measles Virus

Thomas Peebles, MD, working in a laboratory at Boston Children’s Hospital, was asked by lab director John Enders to isolate the virus responsible for measles. Peebles learned of an outbreak at a private school outside of Boston and, after getting permission from the principal, collected blood samples from the sick students, telling each boy: “Young man, you are standing on the frontiers of science.” Peebles attempted for weeks to obtain the virus, and on February 8, succeeded. He collected blood containing the virus from 13-year-old student David Edmonston. Eventually, the collected measles virus would be isolated and used to create a series of vaccines.

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Timeline Entry: 1962

Rubella Virus Isolated

Maurice Hilleman and Eugene Buynak isolated the Benoit strain of rubella virus. They hoped to develop an inactivated virus vaccine, but eventually gave up this idea in favor of an attenuated virus vaccine.

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