The Disease Triangle: The Key to Understanding All Diseases

What is the Disease Triangle:

The first thing I ever learned about plant diseases was a clever tool called the Disease Triangle. The purpose of the disease triangle is to get you thinking about what components are needed for disease formation. At each of the points on the triangle is one of the key components of disease: The host, the pathogen, and the environment.

The best part about the disease triangle is that it can apply to nearly all diseases, not just plant ones. All diseases require a susceptible host, the right pathogen, and optimal environmental conditions to cause disease. Let’s take a look at each of these components one at a time.

Fun Fact: The CDC uses a slightly different name for the disease triangle: The Epidemiologic Triangle. The two triangles both get you to think about disease in the same way. The main difference is that the epidemiologic triangle uses broader terms that cover diseases that are not caused by a pathogen, such as iron toxicity or malnutrition. Mineral toxicity and malnutrition are considered diseases, because they are a departure from normal healthy conditions, yet there are no pathogens involved.

The Host:

The first component needed for disease is a susceptible host. Children and the elderly tend to have weaker immune systems, making them the most susceptible to harmful microbes. However, all humans have some degree of susceptibility, so everyone needs to protect themselves from harmful pathogens.

The host component has a slightly different meaning with regard to plants and livestock. Resistance can be bred or introduced into a crop or farm animal that makes them either immune or less susceptible to a disease. Unlike humans, we can directly select for resistance genes within these organisms. For example, potatoes that are resistant to Late Blight, a disease caused by a destructive fungal-like pathogen, have been selected for over the years in order to prevent a repeat of the Irish Potato Famine.

Fun Fact: Late Blight of Potato is the same disease that caused the Irish Potato Famine in the 1840s. It is not actually a fungus, although it is very similar. Late Blight is instead an Oomycete, which is actually a Protist.

3 Easy Ways to Prevent Disease on the Host Side:

• Staying fit. By exercising and staying fit, we keep our bodies strong. Being out of shape makes it harder for our bodies to fight diseases when we come into contact with them.
• Avoid contact with pathogens. We can avoid risky situations like eating or drinking after someone who is already sick. Sharing utensils, drinking from the same glass, and being lazy while washing dishes are three common ways for disease to spread within a household.
• Get vaccinated. Vaccines work by introducing a weaken version of a pathogen into our system. This weakened pathogen cannot cause disease, but can trigger our immune system into creating antibodies against the pathogen. If this occurs before we get a disease, we build up a strong defense against the pathogen before it arrives.

The Pathogen:

The second component of the disease triangle is the pathogen. Each pathogen has specific hosts that it can and cannot infect, meaning that the pathogen needs to find the correct host in order to cause disease. Human pathogens, for example, do not generally infect plants; just as plant pathogens do not infect animals. If a human pathogen encounters a plant, it cannot infect the plant and cannot cause disease. For example, Salmonella, which can cause food poisoning, infects mammals, but does not seem to cause disease in birds or reptiles. However, pathogens like Salmonella frequently hitch rides from these non-host animals (called vectors) from one host to the next.

2 Easy Ways to Prevent Disease on the Pathogen Side:

• Regulating contamination of vectors. In the United States, the government strictly controls and actively checks for foodborne pathogens within poultry, eggs, and some produce. Insects often vector pathogens as well, so keeping pest populations low is a good strategy. These controls minimize the chance of pathogenic coli or Salmonella from being in the food on our tables.
• Thoroughly cook your food. Each type of meat has a suggested cooking temperature. These temperatures are established to ensure that the meat reaches a temperature that kills the associate pathogens. When we consume raw or undercooked meat, we dramatically increase our likelihood of getting sick.

The Environment:

The last component is the environment. The environment is important to disease formation, because it controls whether the pathogen has a chance at causing the disease or not. For example, downy mildew is a plant pathogen that damages many plant species, including grapes, hops, and herbs. Downy mildew needs moisture in order to spread and infect a plant. Therefore, farmers can keep the plant leaves drier by spreading the plants out within a field and increasing the amount of evaporation. This example demonstrates how the environment can play a large role in whether we see disease or not.

Another example is the beneficial microbes that live on the surface of our skin and in our gut. These microbes have an intimate relationship with us, trading nutrients and helping us stay healthy. An important benefit of this relationship is that these helpful microbes act as a barrier or defense system again pathogens. It turns out that pathogens end up harming these microbes indirectly by hurting the host (us), so there is a strong incentive to keep us safe from pathogens.

2 Easy Ways to Prevent Disease on the Environment Side:

• Clean the door knobs and light switches. This trick took me a while to figure out, but when you think about it, it makes complete sense. Which two things does everyone touch when they enter a room? The door knob and the light switch. If someone is sick in your house, try cleaning these objects until they get better. By cleaning your immediate environment, you can limit the spread of a disease within a household.
• Maintain a healthy diet. A consistent, healthy diet not only benefits your weight, but it also encourages a healthy set of gut microbes. The microbes in our gut adapt to what we eat, whether positively or negatively. By maintaining a consistent and healthy diet, we foster a set of microbes that can protect us from pathogens in our food.

Whether talking about the common cold, tape worms, or Ebola, the disease triangle can be used to better understand which components are needed for disease formation. This knowledge can help us create efficient strategies for prevention and containment of diseases. The next time a family member is sick, think about the disease triangle, and you might prevent the disease from spreading to the rest of the family.

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Questions of the Day:

1. What other ways can you think of to prevent pathogens from causing disease in your house?
2. What is one easy fix that you can make that will protect you from disease?