Classification of Diseases
Senior 1 · Biology
Classification of Diseases
Lesson 3 of 5
25 min
In the same month, two problems appear in a boarding school (an illustrative case). In one dormitory, several students develop itchy, ring-shaped patches on their scalps; they had been sharing combs. Meanwhile, students in different classes fall ill with fever and coughing, one after another, in the crowded dining hall. Both are infections, but they travel by different routes, and a route that is understood can be blocked.
Definition
Transmission: the passing of a pathogen from one host to another.
Reservoir: the place where a pathogen normally lives and multiplies: an infected person, an animal, water or soil.
Carrier: a person who has a pathogen and can pass it on without showing signs of the disease.
Vector: an animal, usually an insect, that carries a pathogen from one host to another. Example: the female Anopheles mosquito, which carries the malaria parasite.
Figure 1: The chain of infection. A pathogen passes from its reservoir to a new host through six links; removing any one link stops the spread.
| Route | How the pathogen travels | Examples | How to block it |
|---|---|---|---|
| Air (droplets and airborne) | tiny drops from coughs, sneezes and talking are breathed in by others | influenza, common cold, tuberculosis, measles, COVID-19 | cover coughs and sneezes; open windows; vaccination; masks when ill |
| Water and food (faecal–oral) | pathogens in faeces get into water or onto food and are swallowed | cholera, typhoid, amoebic dysentery, hepatitis A | latrines; safe (boiled or treated) water; handwashing; covering and cooking food well |
| Vectors | an animal carries the pathogen: inside its body (mosquito) or on its feet and mouthparts (housefly) | malaria (mosquito); diarrhoeal diseases spread by houseflies | bed nets; removing stagnant water; covering food and latrines |
| Contact | touching an infected person, or sharing objects such as combs, towels and clothes | ringworm, scabies, conjunctivitis | do not share personal items; wash; treat cases quickly |
| Body fluids | blood, or fluids during sex or childbirth, pass from an infected person | HIV, hepatitis B | never share needles, razors or toothbrushes; use gloves for blood; testing and treatment |
| Disease | Cause (pathogen) | How it spreads | Prevention |
|---|---|---|---|
| Malaria | Plasmodium (protoctist) | bite of an infected female Anopheles mosquito | treated bed nets; spraying houses; removing stagnant water; early testing and treatment |
| Cholera | Vibrio cholerae (bacterium) | water or food contaminated with faeces | safe water; latrines; handwashing; cholera vaccine in outbreaks |
| Typhoid | Salmonella Typhi (bacterium) | food or water contaminated with faeces, often from a carrier | safe water; food hygiene; handwashing |
| Tuberculosis (TB) | Mycobacterium tuberculosis (bacterium) | air, from the coughs of a person with TB of the lungs | BCG vaccine for babies; ventilation; finding and treating cases |
| Measles | measles virus | air (droplets), very easily | measles vaccine (two doses) |
| Influenza | influenza virus | air (droplets); contaminated hands | covering coughs; handwashing; staying home when ill |
| HIV infection | human immunodeficiency virus | body fluids: sex, blood, mother to baby | abstinence or condoms; never sharing needles; testing; treatment |
| Ringworm | fungi | contact with infected skin or shared combs and towels | not sharing personal items; keeping skin clean and dry |
| Bilharzia (schistosomiasis) | Schistosoma (parasitic worm) | larvae from water snails pass through the skin in fresh water contaminated by urine or faeces | latrines; avoiding washing or swimming in snail-infested water; treatment |
Worked example 1: tracing the ringworm outbreak
In the opening case, 6 of the 40 students in one dormitory had ringworm. (a) What percentage were affected? (b) Name the pathogen group and the route. (c) Which link would you break, and how?
Answer: 15%; a fungal infection spread by contact; stop sharing combs and treat the cases.
Worked example 2: filling in the chain for tuberculosis
Use Figure 1 to describe each link of the chain for TB in the dining-hall case.
Breaking links: a person who has coughed for more than two weeks should be tested; treatment removes the reservoir, open windows weaken the mode of transmission, and covering coughs blocks the portal of exit.
Warning
Common mistakes. (1) "Malaria spreads from person to person." It needs the mosquito; you cannot catch it by touching or living with someone who has malaria. (2) "HIV spreads by sharing food, hugging or mosquito bites." It does not; it spreads only through certain body fluids. (3) "A person who looks well cannot spread disease." Carriers and people in the early stages of an infection can pass it on.
How do germs spread (and why do they make us sick)? (TED-Ed). Watch for the different routes germs take from one person to another, and how each can be blocked.
Tip
Memory hook: "AWVCB", Air, Water and food, Vectors, Contact, Body fluids. In an exam about any infectious disease, give three things in order: the pathogen, its route, and a barrier that blocks that route.
Practice: how infectious diseases spread
Key points
Sources and credits
Written by Ganzaa: the school case, Figure 1, both tables, the worked examples (illustrative figures) and the practice questions.
