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B5.3 Preventing Infections

GCSE Combined Science · AQA · Chapter B5: Communicable Diseases

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: how Semmelweis changed medicine, and the four main ways to reduce or prevent the spread of communicable diseases. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe how Semmelweis showed that hand-washing reduced deaths from infection
  • Explain how hygiene, isolation, destroying vectors and vaccination reduce the spread of disease
  • Calculate percentages and percentage changes in infection data
  • Evaluate methods of controlling the spread of a disease

Key words

  • Hygiene: Keeping people, equipment and places clean to remove or kill pathogens, e.g. hand-washing.
  • Isolation: Keeping an infected person, animal or plant away from others so the pathogen cannot spread.
  • Vector: An organism that carries a pathogen from one host to another, e.g. a mosquito.
  • Insecticide: A chemical that kills insects. Used to destroy insect vectors.
  • Vaccination: Giving a dead or inactive form of a pathogen so that the body becomes immune to the disease.

Learning from history: Ignaz Semmelweis

In the 1840s Ignaz Semmelweis worked in a hospital in Vienna. Many mothers died of childbed fever after giving birth. Far more died on the ward run by doctors and medical students than on the ward run by midwives.

The doctors often came straight from examining dead bodies to deliver babies, without washing their hands. Semmelweis suggested they were carrying something from the bodies to the mothers. He made doctors wash their hands in chlorinated lime solution (a disinfectant), and the death rate fell sharply.

Many doctors ignored him. Microorganisms had not yet been linked to disease, he could not show what was being passed on, and doctors did not like being told they were causing deaths. Later, scientists showed that microorganisms cause disease and hospitals began to use antiseptics.

Four ways to stop diseases spreading

  • Hygiene; How it works Removes or kills pathogens before they reach a new host.; Examples Hand-washing; disinfecting surfaces and kitchen boards; cooking food thoroughly; using tissues; clean water and sewage treatment
  • Isolating infected individuals; How it works Keeps infected people, animals or plants away from healthy ones.; Examples Hospital isolation rooms; staying at home when ill; removing infected plants
  • Destroying or controlling vectors; How it works Stops the vector carrying the pathogen to a new host.; Examples Draining standing water and spraying insecticide to kill mosquitoes; mosquito nets
  • Vaccination; How it works People (or animals) become immune, so they cannot be infected or pass the pathogen on.; Examples MMR vaccine for children; poultry vaccinated against Salmonella

Key fact: If most people in a population are vaccinated, the pathogen has few hosts to spread through. This protects people who are not vaccinated too, and is called herd immunity (more in B6.1).

Working with infection data

Worked example: percentage decrease

A care home had 72 infections in the year before a new hand-gel rule. In the year after, it had 27.
Decrease = 72 − 27 = 45.
Percentage decrease = 45 ÷ 72 × 100 = 62.5%.

Answer: A 62.5% decrease

When you evaluate data, ask: Was anything else changed at the same time? Was the number of patients the same each year? Is one year of data enough to be sure? Compare percentages if the numbers of people are different.

Exam tip: Percentage change = (new value − original value) ÷ original value × 100. A negative answer means a decrease.

Common mistakes

  • Saying Semmelweis knew about bacteria. He did not; he only knew something was passed on by unwashed hands.
  • Mixing up isolation (separating the ill) with vaccination (making healthy people immune).
  • Dividing by the new value instead of the original value when working out a percentage change.
  • Saying a vector is the pathogen. The mosquito is the vector; the protist it carries is the pathogen.

Revision questions

Try these, then check your answers below each one.

Question 1. In the 1840s Ignaz Semmelweis noticed that many mothers on a doctors' ward died after childbirth. What did he ask the doctors to do? (Choose from: Wear gloves and masks, Wash their hands in a chlorinated lime solution, Give the mothers antibiotics, Keep the windows shut.)

Answer: Wash their hands in a chlorinated lime solution

Working: Semmelweis made the doctors wash their hands in chlorinated lime (a disinfectant) before examining patients. The death rate fell sharply.

Question 2. The bar chart shows illustrative data for a ward before and after hand-washing was introduced. Calculate the percentage decrease in the death rate.

Answer: 75 %

Working: (12.0 − 3.0) ÷ 12.0 × 100 = 75%.

Question 3. True or false? Semmelweis was able to show doctors which microorganism caused childbed fever.

Answer: False

Working: False. Bacteria had not yet been linked to disease. Semmelweis could only show that hand-washing reduced deaths, which is one reason many doctors ignored him.

Related lessons

Continue in the full course

This lesson is section B5.3 (AGCSC0503) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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