Infectious and noninfectious diseases
At Valeria's school, half the students stayed home for a week because of the flu. Her uncle Mateo, on the other hand, has had diabetes for years and nobody in the family ever "caught" it. Both are diseases, but one jumps from person to person and the other does not. Why? Knowing the answer helps you know how to prevent each one.
Infectious or noninfectious disease
If two people share a bedroom and one has the flu, it is very likely that the other will get sick too. If one has diabetes, the other won't "catch" it no matter how much time they spend together. What's different?
An infectious disease is caused by a pathogen (a microorganism) that enters the body and multiplies. A noninfectious disease is not caused by a pathogen: it depends on genetics, habits, the environment, or how the body works.
Careful: some infectious diseases are contagious from person to person (flu, measles) and others are not. Tetanus, for example, happens when spores from the environment (soil, rust) get into a wound, but a sick person does not pass it to their family. Infectious and contagious are not synonyms.
Among the noninfectious diseases are some of the most common ones in Mexico today: obesity, diabetes, high blood pressure, asthma, and heart disease.
Explain the fundamental difference between an infectious disease and a non-infectious disease, and classify the following into the two groups: flu (influenza), diabetes,…
Explain the fundamental difference between an infectious disease and a non-infectious disease, and classify the following into the two groups: flu (influenza), diabetes, tuberculosis, asthma, measles, high blood pressure (hypertension).
Infectious agents
For centuries people thought fevers came from "bad air." The microscope revealed something else: tiny living things settled in diseased tissue. Which ones are able to cause us an infection?
General characteristics of the two most famous ones:
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Bacteria: single-celled prokaryotic living things (no nucleus), with a cell wall. They reproduce on their own by dividing, very quickly if there is warmth and food.
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Viruses: much smaller, they have no cells (they are acellular). They contain only genetic material inside a coat, and they can multiply only inside the cells of a living thing.
Why do infections spread so fast?
A reasonable hypothesis: because pathogens multiply very quickly (a bacterium can divide several times an hour) and because they pass easily from one person to another when people live together, share air, objects, or food. A hypothesis is tested against evidence: that is why we consult scientifically supported sources, such as Mexico's Ministry of Health (Secretaría de Salud) or the WHO, which record how many cases appear and how they decrease when the routes of transmission are cut off.
Antibiotics and resistance: natural selection in action
When someone takes an antibiotic, almost all the bacteria die, but sometimes one survives that, by chance, is more resistant. If the treatment is stopped too early, those remaining bacteria multiply and their descendants inherit the resistance. This is natural selection: the antibiotic "chooses" the most resistant ones. The important point is that the bacteria develop the resistance, not the person's body.
Name the four main types of agents that cause infectious diseases (viruses, bacteria, fungi and protozoa) and cite one disease caused by each type.
Name the four main types of agents that cause infectious diseases (viruses, bacteria, fungi and protozoa) and cite one disease caused by each type.
Antibiotics cure bacterial infections, but are useless against flu or the common cold. Explain why, taking into account which agent causes each type of disease, and…
Antibiotics cure bacterial infections, but are useless against flu or the common cold. Explain why, taking into account which agent causes each type of disease, and reason what danger there is in taking non-prescription antibiotics "just in case".
How infections are transmitted
Some vectors, such as the Aedes aegypti mosquito, transmit pathogens between people: that is how dengue, Zika, and chikungunya spread. Other routes are sexual and bloodborne; each one calls for different measures.
In the diagram, a sick person (left, surrounded by microorganisms) can transmit their infection to a healthy person (right) along three routes labelled with ?. Write the…
In the diagram, a sick person (left, surrounded by microorganisms) can transmit their infection to a healthy person (right) along three routes labelled with ?. Write the name of the transmission route that each path represents and give an example of a disease for each one.
A class of 24 pupils goes on a field trip and the next day 6 classmates fall ill with gastroenteritis after sharing sandwiches. Calculate what percentage of the class…
A class of 24 pupils goes on a field trip and the next day 6 classmates fall ill with gastroenteritis after sharing sandwiches. Calculate what percentage of the class fell ill, explain which transmission route is the most likely, what hygiene measures would have prevented it, and why not everyone who ate necessarily fell ill.
In a flu outbreak at a school with $400$ students, $60$ become infected in the first week. Without preventive measures, the number of weekly infections doubles; with…
In a flu outbreak at a school with $400$ students, $60$ become infected in the first week. Without preventive measures, the number of weekly infections doubles; with measures (hand hygiene, ventilation, and staying at home with symptoms), it only increases by half each week. Calculate how many new infections there would be in the second week in each scenario, compare the evolution of the outbreak in both cases, and reason why it is said that the measures "flatten the curve" even if they do not prevent all infections.
Noninfectious diseases and their risk factors
Obesity and type 2 diabetes are good examples: they are linked to a diet rich in ultra-processed foods, sugars, and fats, and to a sedentary lifestyle. Neither one is contagious, but both can be largely prevented with habits: eating vegetables and fruit, drinking water instead of soda, moving every day.
Cardiovascular diseases are non-infectious, yet they are closely related to lifestyle habits. Explain which modifiable risk factors favour them and propose four…
Cardiovascular diseases are non-infectious, yet they are closely related to lifestyle habits. Explain which modifiable risk factors favour them and propose four preventive measures that a young person can adopt.
A patient who has been prescribed antibiotics for 8 days for bacterial pharyngitis decides to stop taking them on the fourth day because they no longer have a fever or…
A patient who has been prescribed antibiotics for 8 days for bacterial pharyngitis decides to stop taking them on the fourth day because they no longer have a fever or pain, arguing: "If I already feel fine, the fewer pills I take, the less my body will get used to the medication and the better it will respond in the future." Identify the two scientific errors in their reasoning (one regarding which organism resistance acts upon and another about what happens to the population of bacteria still remaining in their throat) and justify, in terms of natural selection, why stopping the treatment early increases the risk of a relapse with bacteria that are harder to eliminate.
Two identical twin sisters share a genetic mutation that increases the risk of developing a certain type of cancer. One of them states: "Since the cause is in our DNA,…
Two identical twin sisters share a genetic mutation that increases the risk of developing a certain type of cancer. One of them states: "Since the cause is in our DNA, our fate is sealed: we will get sick no matter what we do with our lifestyle, and what is more, our flatmates could catch it if they live with us." Refute both parts of her statement by distinguishing between genetic predisposition and environmental triggering factor, and justify why cancer is not considered an infectious disease even though there are external agents (such as tobacco or certain viruses) that promote its onset.
Back to the beginning: the flu is infectious (it is caused by a virus that travels through the air), while uncle Mateo's diabetes is noninfectious: it depends on metabolic and genetic causes and on habits, and that is why it is never contagious.