5.5 Preventing Infections

A free GCSE Biology lesson for AQA (8461) 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, controlling vectors and vaccination reduce the spread of disease
• Calculate 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 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.
• Vaccination: Giving a harmless form of a pathogen so that the body becomes immune to the disease.
• Epidemic: A large outbreak of a disease in one area at the same time.

Learning from history: Ignaz Semmelweis

In the 1840s Ignaz Semmelweis worked in a hospital in Vienna. Many mothers died of childbed fever soon 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 dissecting 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 a chlorinated lime solution (a disinfectant). The death rate fell sharply.

Many doctors refused to believe him. Bacteria 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, Louis Pasteur showed that microorganisms cause disease, and Joseph Lister began using antiseptics in surgery.

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; cooking food well; covering coughs and sneezes; clean water and sewage treatment
• Isolation; How it works Keeps infected individuals away from healthy ones.; Examples Hospital isolation rooms; staying home when ill; quarantine of imported animals; removing infected plants
• Destroying or controlling vectors; How it works Stops the vector carrying the pathogen.; Examples Draining standing water and spraying insecticide for mosquitoes; mosquito nets; controlling insect pests on crops
• Vaccination; How it works People become immune, so the pathogen cannot infect them or be passed on.; Examples Measles (MMR) vaccine for children; poultry vaccinated against Salmonella

Key fact: If enough people are vaccinated, the pathogen has few hosts to spread through, which also protects people who cannot be vaccinated. This is called herd immunity (more in chapter 6).

Plant diseases are controlled in similar ways: removing and burning infected plants, cleaning tools, controlling insects such as aphids, rotating crops and growing resistant varieties.

Working with infection data

Worked example: percentage decrease

A hospital had 135 infections in the year before a new hand-gel rule. In the year after, it had 54.
Decrease = 135 − 54 = 81.
Percentage decrease = 81 ÷ 135 × 100 = 60%.

Answer: A 60% decrease

When evaluating 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?

Exam tip: Percentage change = (new value − old value) ÷ old 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. What did Ignaz Semmelweis ask doctors to do? (Choose from: Wear face masks, Wash their hands in a chlorinated lime solution, Give patients antibiotics, Isolate every mother.)

Answer: Wash their hands in a chlorinated lime solution

Working: Semmelweis made doctors wash their hands in a chlorinated lime solution (a disinfectant). Deaths from childbed fever fell sharply.

Question 2. True or false? Semmelweis knew that bacteria caused childbed fever.

Answer: False

Working: False. Bacteria had not been linked to disease. He only knew that something was passed on by unwashed hands. Pasteur showed microorganisms cause disease later.

Question 3. A clinic had 120 infections one year and 42 the next. What was the percentage decrease?

Answer: 65 %

Working: (120 − 42) ÷ 120 × 100 = 78 ÷ 120 × 100 = 65%.

Related lessons

• Previous lesson: 5.4 Preventing Bacterial Growth
• Next lesson: 5.6 Viral Diseases
• In this chapter: 5.1 Health and Disease
• In this chapter: 5.2 Pathogens and Disease
• In this chapter: 5.3 Growing Bacteria in the Lab

Continue in the full course

This lesson is section 5.5 (AGCBH0505) of our free GCSE Biology 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 Biology course.