Exercises: Infectious and non-infectious diseases

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).

Name the four main types of agents that cause infectious diseases (viruses, bacteria, fungi and protozoa) and cite one disease caused by each type.

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.

Explain what a microorganism is and reason, with an example, why not all microorganisms are harmful to humans.

Name three routes by which infectious diseases are transmitted and cite one disease transmitted by each one.

Explain what contagion is and what difference there is between having been infected and having developed the disease.

State whether these statements are true or false and justify your answer: a) the common cold is caused by exposure to cold; b) diabetes is not passed from one person to another; c) washing your hands frequently reduces the transmission of infections.

Explain what an epidemic is and cite two general measures that help to slow it down.

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.

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".

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.

Drinking water and sewers reduced diseases such as cholera. Explain which transmission route these infrastructures cut, and reason why millions of people still fall ill in areas that lack them.

Head lice spread easily in schools. Explain what type of transmission they have, why their appearance does not indicate a lack of hygiene, and which measures slow their spread in the classroom.

In hospitals, hand hygiene and the isolation of certain patients are taken to the extreme. Explain what hospital-acquired infections are, why admitted patients are more vulnerable, and what role antibiotic resistance plays in them.

Explain what the risk factors of a disease are and classify these examples into modifiable and non-modifiable: age, diet, smoking, genetic make-up, sedentary lifestyle.

Tooth decay is one of the most common and, at the same time, most preventable infectious diseases. Explain which bacteria cause it, which food favours their action on the teeth, and which three habits prevent it.

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.

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.

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.

After a field trip to the countryside, an isolated case of bubonic plague is detected in a person, caused by the bite of a wild rodent flea infected with the bacterium Yersinia pestis. An alarm goes off on social media with the headline: "The Black Death of the 14th century is back: we are on the verge of a catastrophe that will ravage Europe!". Write a scientific response refuting that alarm:

1. a) Justify three key differences (in scientific knowledge, hygienic-sanitary conditions, and available treatments) between the 14th century and today that prevent this isolated case from leading to a similar pandemic.
2. b) If effective antibiotics against this bacterium exist today, why do health authorities still immediately isolate the patient and trace all their close contacts?

A student returns from a trip to a tropical area and a few days later is diagnosed with malaria. A group of families asks the school management not to allow the student to attend classes for two weeks "out of fear that they will infect others by coughing or sharing school supplies."

1. a) Based on the type of microorganism causing it and its transmission route, write a scientific explanation to reassure the families, justifying why direct transmission between classmates in the classroom is biologically impossible.
2. b) If a plague of common mosquitoes (Culex pipiens) were to proliferate in the high school gardens, would there be a risk of them biting the infected student and transmitting malaria to the rest of the school? Justify your answer by applying the concept of a specific biological vector.

On an intensive livestock farm, continuous low doses of antibiotics were mixed into feed to accelerate the fattening of healthy animals. Months later, the local hospital sees an increase in severe human infections caused by bacteria resistant to those same drugs. The farmer states: "My animals have never been in contact with hospital patients, so our farm has nothing to do with what happens there."

1. a) Refute the farmer's argument by explaining the biological mechanism by which the preventive or mass use of antibiotics in animals selects for resistant bacteria, and predict two possible routes through which these bacteria can reach the human population.
2. b) Design two complementary measures (one applicable in livestock production and another in the hospital setting) to curb the proliferation of these "superbugs."

Imagine you are part of the scientific advisory committee facing the emergence of a new airborne respiratory virus. It is known that the pathogen has an asymptomatic incubation period of $4$ days (during which the person can already transmit it), remains active in the air of poorly ventilated indoor spaces for more than an hour, and there is still no vaccine or specific treatment.

1. a) Design a protocol with three non-pharmacological preventive measures to apply in a secondary school and justify, for each of them, which link in the chain of transmission (source of infection, transmission mechanism or reservoir, or susceptible host) you are blocking.
2. b) If after several months a vaccine is approved that reduces severe cases by $95$ % but only reduces the ability to become infected and transmit the virus by $30$ %, predict what would happen if all ventilation and mask measures were suddenly removed as soon as the vaccination campaign begins.

During a school vaccination campaign against the human papillomavirus (HPV), a leaflet states: "This vaccine prevents certain types of cancer". A student comments: "That makes no sense: vaccines only protect against infectious microbes, and cancer is an uncontrolled multiplication of our own cells, not a contagious infection."

1. a) Help the student resolve this apparent paradox: explain the fundamental difference between an infectious and a non-infectious disease, and justify why cervical cancer is classified as non-infectious even though its main trigger is a transmissible virus.
2. b) Explain how the vaccine acts on the immune system at the time of its administration to successfully prevent the appearance of a tumour that could take $20$ years to develop.