4 Treatment and Prevention

Reading 4

Mimicking an Infection

The purpose of this reading is to have students explore how vaccines work to activate the immune response against specific viruses and bacteria. Students should recognize that vaccines mimic an actual infection and provide protection against infection in a similar fashion without the dire consequences of the disease. In Module 3 – Viruses Go “Viral”, students learn about the impact of vaccines on infectious diseases and consider the implications and ethics of mandatory vaccinations.

How many vaccinations have you endured? Can you remember being vaccinated as a toddler or young child? Perhaps your family elected not to have you vaccinated. Exactly what happens when you receive a vaccine? First, a historical perspective.


Smallpox outbreaks and epidemics have occurred throughout history, leaving disfigurement and death in its wake (see Timeline of Infectious Diseases). Caused by the Variola virus, a large DNA virus, smallpox is extremely contagious, spreading through a population through virus-laden droplets released by the coughing and sneezing of infected individuals. During the 18th century smallpox killed an estimated 400,000 people in Europe every year. Even as recently as 1967 an estimated 15 million people contracted the disease and of those, two million died.

Edward Jenner

But all that has changed, thanks to the work of Edward Jenner, an English physician practicing in the late 1700s. Jenner observed that milkmaids rarely contract smallpox. He knew that milkmaids were often in contact with cows infected with a virus that caused a disease similar to smallpox.

Jenner hypothesized that contact with cowpox protected milkmaids from smallpox without causing any symptoms. He tested his hypothesis by scraping material from cowpox lesions and scratching this material into the skin of healthy individuals including his own son. This early form of vaccination proved effective, providing protection against smallpox infection.

This video by Dartford Technology College shows the story of Edward Jenner and his discovery.

Modern Vaccines

Modern day smallpox vaccines are made from Vaccinia, a virus closely related to the cowpox virus. Extensive vaccination programs around the world led the World Health Organization to announce in 1980 that smallpox had been conquered and eradicated from the face of the Earth, the only infectious disease to ever achieve that status. Since Jenner’s discovery, children can now receive vaccine protection for 16 infectious diseases.

How Vaccines Work

Modern day vaccines are made using different types of materials.

Vaccines used to protect against measles, mumps, rubella, chickenpox, and rotavirus contain live viruses that cause the disease that have been weakened or altered in such a way that they do not cause an illness.

Inactivated or killed bacteria are used in vaccines that protect against polio, hepatitis A and rabies.

A fragment of a virus or bacterium is often sufficient to protect against infection. Vaccines against hepatitis B, influenza, pertussis (whooping cough), meningitis, pneumonia, and human papilloma virus contain just a fragment of the infectious agent that causes the disease.

All of these substances can serve as antigens that activate the immune system and respond as though an actual infection by the infectious agent was taking place.

As you learned in Reading 3 On Guard, once the immune system has responded to a viral or bacterial infection, the body remains protected against a second infection by that specific pathogen by memory B cells that produce protective antibodies.

When the body is first infected by a pathogen or vaccinated against this pathogen, the immune system is activated as you read in Reading 3.

It is a complex process that takes time so if you are undergoing an actual infection you may get quite sick until the immune response kicks in and clears your body of the pathogen.

If you are vaccinated against the pathogen your immune system again takes time to respond to the antigen in the vaccine but in this case you do not get sick because no actual infection occurs.

In both cases, after an actual infection and vaccination, your body is “primed” by memory B cells and is ready to fight off a second infection.

It is rare to be reinfected with chicken pox or measles if you had the disease once before because memory B cells producing antibodies to that pathogen are in your bloodstream and ready to attack.

Vaccination provides the same protection as an actual infection without the misery of the actual disease.

The following video, How a Vaccine Works, reviews the immune response and how vaccines work to protect against infectious diseases.

Additional Resources for Module 4 Reading 4

History in the Headlines: The Rise and Fall of Smallpox, history.com

This website provides information about the history of smallpox and the discovery of the smallpox vaccine.

How Vaccines Are made, The College of Physicians of Philadelphia

This slideshow provides a description of how vaccines are made.

The History of Vaccines, The College of Physicians of Philadelphia

The History of Vaccines website provides articles, activities, lesson plans, videos, and an interactive game related to the immune system and vaccines.