How does an infectious disease spread? HIV simulation

Author(s): SEP staff

Lesson Overview

Grade level(s):

Middle School (6-8), High School (9-12), Grade 7, Grade 8, Grade 9, Grade 10, Grade 11


Biology/Life Science


Infectious diseases; health

Big ideas(s):

This activity will simulate the spread of an infectious disease through a population and how behavior can affect the risk of getting infected.

Vocabulary words:

Infectious disease, bacteria, virus, pathogen, transmission, indicator, airborne/bloodborne pathogens, HIV, AIDS

What you need:

  • HIV Simulation Kit K105 from SEP
    which contains:
    - numbered vials; test tubes would work as well - plastic droppers (one for each student) - 25% Potassium Hydroxide Solution (KOH); CAUTION! IRRITANT!! - Phenolphtalein (pH Indicator) - Vinegar (to wash vials and droppers at the end of the lab)
    - behavior cards (see attachment)
    - disease cards (see attachment)


Whole class for activity; independent for graphing part


regular classroom that allows for students to move around

Time needed:

One class period

Author Name(s): 
SEP staff

Students will simulate the exchange of bodyfluids and then test whether they got infected with a disease. This activity will show how one person who is infected with a disease can infect other people, who in turn infect others. Students will be able to see how behavior can effect their risk of getting infected.

The lesson plan was inspired by many educators. Thanks to Lance Powell at June Jordan HS in San Francisco, Jennifer Doherty and Dr. Ingrid Waldron, University of Pennsylvania

Learning goals/objectives for students: 

Students will be able to understand how infectious disease spread through a population. 
Students will be able to identify behavior that increase or decrease the risk of infections.

Content background for instructor: 

In order for the students to predict the number of infections after 4 and 5 interactions, students should notice that the number of infections approximately double with each additional interaction.

Number of         Previously        Newly            Total #
interactions       infected           infected      infections

1              Student #1     Student #2          2

2             Student #1     Student #3           4
Student #2     Student #4

3               Student #1     Student #5         8
Student #2      Student #6
Student #3      Student #7
Student #4      Student #8

This doubling in each generation follows the pattern of an logistic growth curve (S-curve). In the beginning the curve increases exponentially, but then levels out. The same pattern can be observed within this activity. As the number of infected student increases, it become increasingly more likely that an infected student interacts with another student that already has been infected. As a result, the number of new infections slows down.


(The following information was found at


How do infectious diseases spread?

Direct contact
The easiest way to catch most infectious diseases is by coming in contact with someone who has one. This "someone" can be a person, an animal or, for an unborn baby, its mother. Three different ways infectious disease can be spread through direct contact are:

  • Person to person. The most common way for infectious disease to spread is through the direct transfer of bacteria, viruses or other germs from one person to another. This can occur when an individual with the bacterium or virus touches, coughs on or kisses someone who isn't infected. These germs can also spread through the exchange of body fluids from sexual contact or a blood transfusion.
  • Animal to person. Your household pet might seem harmless, but pets can carry many germs. Being bitten or scratched by an infected animal can make you sick and, in extreme circumstances, could even cause death. Handling animal waste can be hazardous, too. You can become infected by scooping your cat's litter box or by cleaning bat or mouse droppings in your house or garage.
  • Mother to unborn child. A pregnant woman may pass germs that cause infectious diseases on to her unborn baby. Germs can pass through the placenta, as is the case of the AIDS virus and the toxoplasmosis parasite. Or you could pass along germs during labor and delivery, as is the case for a mother infected with group B streptococcus.

Indirect contact
Disease-causing organisms can also be passed along by indirect contact. Many germs can linger on an inanimate object, such as a tabletop, doorknob or faucet handle. When you touch the same doorknob grasped 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.

Infectious diseases spread through the air

Droplet transmission
When you cough or sneeze, you expel droplets into the air around you. When you're sick with a cold or the flu — or any number of other illnesses — these droplets contain the germ that caused your illness. Spread of infectious disease in this manner is called droplet spread or droplet transmission.

Droplets travel only about three feet because they're usually too large to stay suspended in the air for a long time. However, if a droplet from an infected person comes in contact with your eyes, nose or mouth, you may soon experience symptoms of the illness. Crowded, indoor environments may promote the chances of droplet transmission — which may explain the increase in respiratory infections in the winter months.

Particle transmission
Some disease-causing germs travel through the air in particles considerably smaller than droplets. These tiny particles remain suspended in the air for extended periods of time and can travel in air currents. If you breathe in an airborne virus, bacterium or other germ, you may become infected and show signs and symptoms of the disease. Tuberculosis and SARS are two infectious diseases usually spread through the air, in both particle and droplet forms.

Infectious diseases spread through vectors and vehicles

Bites and stings
Some germs rely on insects — 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, and deer ticks may carry the bacterium that causes Lyme disease.

The vector-borne spread of germs happens when an insect that carries the germ on its body or in its intestinal tract lands on you or bites you. The germs travel into your body and can make you sick. Sometimes the germs that cause infectious disease need the insect for specific biological reasons. They use the insect's body to multiply, which is necessary before the germs can infect a new host.

Food contamination
Another way disease-causing germs can infect you is through food and water. Common-vehicle transmission allows the germs to be spread to many people through a single source. Food is the vehicle that spreads the germs and causes the illness. For instance, contamination with Escherichia coli (E. coli) is common. E. coli is a bacterium present in certain foods — such as undercooked hamburger or unwashed fruits or vegetables. When you eat foods contaminated with E. coli, chances are you'll experience an illness — also commonly referred to as food poisoning.

Prevent the spread of infectious diseases

Decrease your risk of infecting yourself or others:

  • Wash your hands often. This is especially important before and after preparing food, before eating and after using the toilet.
  • Get vaccinated. Immunization can drastically reduce your chances of contracting many diseases. Make sure to keep your recommended vaccinations, as well as your children's, up-to-date.
  • Use antibiotics sensibly. Only take antibiotics when necessary. And if they're prescribed, take them exactly as directed — don't stop taking them early because your symptoms have abated.
  • Stay at home if you have signs and symptoms of an infection. Don't go to work if you're vomiting, have diarrhea or are running a fever. Don't send your child to school if he or she has these signs and symptoms, either.
  • Be smart about food preparation. Keep counters and other kitchen surfaces clean when preparing meals. In addition, promptly refrigerate leftovers — don't let cooked foods remain at room temperature for an extended period of time.
  • Disinfect the 'hot zones' in your home. These include the kitchen and bathroom — two rooms that can have a high concentration of bacteria and other infectious agents.
  • Practice safe sex. Use condoms if you or your partner has a history of sexually transmitted diseases or high-risk behavior — or abstain altogether.
  • Don't share personal items. Use your own toothbrush, comb or razor blade. Avoid sharing drinking glasses or dining utensils.
  • Travel wisely. Don't fly when you're ill. With so many people confined to such a small area, you may infect other passengers in the plane. And your trip won't be comfortable, either. Depending on where your travels take you, talk to your doctor about any special immunizations you may need.
  • Keep your pets healthy. Bring your pet to a veterinarian for regular care and vaccinations. Feed your pet a healthy diet and keep your pet's living area clean.


Getting ready: 

If you don't have the kit from SEP, print out and photocopy enough sets of the behavior cards (one for each student) and disease cards (one set for each pair/group of students). Put sets of disease cards in zip lock bags for easy distribution.

For each round of interactions you will need to do the following preparations:

1. Fill one vial half-way with Potassium Hydroxide solution. Record the number of the vial.
2. Fill the rest of the vials half-way with water.

Lesson Implementation / Outline


Assess students prior knowledge by asking them what diseases they know and how one can get those diseases. Hand-out sets of disease cards and ask students to sort them into any categories that they can think of. If they are not familiar with a disease, they can put it in an "unsure" category. Challenge students to try out more than one criteria for grouping the diseases. After students had time to do their categorizing with their group, have students share out their ideas.

Here are some potential categories that students can come up with:

  • Non-communicable (not infectious): Sickle-cell anemia, Leukemia
  • Communicable (infectious): Herpes, Measles, Mumps, West Nile, Flu, Hepatitis, Leukemia, Malaria, Lyme disease, Rabies, HIV
  • Sexually transmitted: Herpes, Hepatitis (B), HIV
  • Viral: Herpes, Measles, Mumps, West Nile, Flu, Hepatitis, Rabies, HIV
  • Bacterial: Lyme disease
  • Transmitted by animals: West Nile, Malaria, Lyme disease, Rabies
  • You can be vaccinated for: Measles, Mumps, Flu

Point out, that a lot of these disease can spread from person to person and discuss the different ways that can happen.

Tell students that they will simulate the spread of a disease that requires the exchange of bodily fluids, such as HIV.

  1. Each student obtains a vial containing a clear liquid and a dropper. Tell students that each vial represents their body and that one student is "infected" with a contagious disease. It is unknown to the students who that person is.
  2. Students will now interact with a partner and simulate the exchange of body fluids. Students will move around the classroom and find a partner to interact with. Both   partners will fill up their dropper with liquid from their vial and place 5 drops into the vial of their partner. Stress that students must NOT dip their droppers into their   partner's vial, but rather let the liquid drop in to avoid contamination.
  3. Students then empty any remaining liquid back into their own vial and use the dropper to gently mix it.
  4. Have students repeat the process with another partner and then return to their seat.
  5. Students guess how many students got infected through the past two interactions.
  6. Each students will test their vial now for the presence of the disease by placing 1-2 drops of the indicator (phenolphthalein) into their vial. If they are infected their liquid will turn bright pink.
  7. Ask the students that are infected to raise their hand. Count and have students record the number of infections.
  8. Have students do another round of interactions, again beginning with only one student being infected. Use a new set of vials for this. In this round, students will interact with three different students.
  9. Again have students estimate the number of infections, have students test their vials and then count the actual number of infections again.
  10. Have students graph the number of infections with increasing number of interactions (see students worksheet) and have them estimate the number of infections after 4 and 5 interactions. Depending on the level of the students, they can use the graph grid provided or set up their own. Younger students might benefit from teacher modeling the graphing of the data.

Extension activity:

In this part students will receive behavior cards that will determine their sexual behavior (monogamous, polygamous, promiscuous, one night stand)

  1. Randomly, hand one behavior card to each student. The most interesting results occur, when the person that has the infected vial in the beginning has the promiscuous or polygamous card.
  2. Have students interact with each other according to their behavior card.
  3. Allow a pre-determined time for the interactions (about 4 minutes or so). Then have students return to their seat and test their vials again.
  4. Have students report out. Record how many students were in each behavior group and how many of them ended up with the infection.
  5. Have students discuss the results.
Wrap-up / Closure: 

Ask students how an airborne disease would spread differently and why. Discuss ways of preventing "catching" an airborne disease.

Extensions and Reflections

Extensions and connections: 

If students record the names or the vial number of the students that they interacted with, they can create a tree of infections, to help them figure out who infected them and whom they subsequently infected.

Behavior cards.doc40.5 KB
Student instructions.doc190.5 KB
tree of transmission.doc32 KB
diseases_cards.doc33.5 KB
NGSS Science and Engineering Practices
NGSS Science and Engineering Practices: 
NGSS Crosscutting Concepts
NGSS Crosscutting Concepts: 

Standards - Grades 9-12 Biology

c. Students know how vaccination protects an individual from infectious diseases.
d. Students know there are important differences between bacteria and viruses with respect to their requirements for growth and replication, the body's primary defenses against bacterial and viral infections, and effective treatments of these infections.
e. Students know why an individual with a compromised immune system (for example, a person with AIDS) may be unable to fight off and survive infections by microorganisms that are usually benign.


interesting exercise

Interesting exercise, we did something similar in my high school health class. Reading through the tips to reduce risk of infection, I can't believe some people share toothbrushes or utensils. My friends call me a germophobic for not sharing my personal items with them. Washing hands is also very important, the same friends say I have OCD for washing my hands before eating! The CDC website has some tips on how to wash hands properly, which I always follow: