Viral life cycles and the pros and cons of vaccination

Viruses can only reproduce inside living cells. Some destroy the cell straight away; others hide in its DNA first.

Practise this lesson free (6 questions)

The lytic pathway

1. The virus attaches to a host cell and injects its genetic material.
2. The host cell is made to copy the viral genetic material and make viral proteins.
3. New viruses are assembled inside the cell.
4. The cell bursts (lysis), releasing many new viruses to infect other cells.

The lysogenic pathway

The viral genetic material is inserted into the host cell's DNA.
It is copied every time the cell divides, but no new viruses are made, so the virus is dormant (inactive) and causes no symptoms.
Later, a trigger can make the viral DNA leave the host DNA and enter the lytic pathway.

Immunisation: advantages and disadvantages

Advantages: protects individuals from serious diseases; if most people are immunised, herd immunity protects those who cannot be (such as very young babies); some diseases can be wiped out (smallpox was eradicated).
Disadvantages: vaccines can cause side effects, usually mild (a sore arm or fever) and rarely serious; they do not always give full protection; some people cannot have certain vaccines.
Worked example

Explain why a person infected by a virus in the lysogenic pathway may have no symptoms for a long time.

  1. The viral DNA is inserted into the host DNA.
  2. It is copied when the cell divides, but no new viruses are made.
  3. No cells burst, so there is no damage yet.

Answer: The virus is dormant: its DNA sits in the host DNA and is copied, but no new viruses are made and no cells burst until it enters the lytic pathway.

Key idea

Lytic: attach, inject, copy, assemble, burst. Lysogenic: viral DNA joins the host DNA and stays dormant, then later enters the lytic pathway. Vaccines protect people and give herd immunity, but can have side effects.

Key words
Vaccination
Giving a dead, weakened or harmless form of a pathogen (or its antigens) so the body makes memory cells without becoming ill.
DNA
Deoxyribonucleic acid: a double helix of two strands of nucleotides that carries the genetic code. Bases pair A with T and C with G.
Virus
A non-living particle made of genetic material in a protein coat. It can only reproduce inside a host cell.

All GCSE and International GCSE Biology key terms and definitions →

Quick check

Try two quick questions

See if it has sunk in. Answers are marked straight away.

Extra practice

2 more questions

Try each one, then tap to check.

  1. In the lytic pathway, what happens after new viruses are assembled?

    1. The virus dies
    2. The host cell divides normally
    3. The virus DNA joins the host DNA
    4. The host cell bursts and releases them
    Show the answer

    Answer: The host cell bursts and releases them

    Lysis releases viruses.

  2. What is the first step of the lytic pathway?

    1. Viral DNA is copied
    2. New viruses are assembled
    3. The virus attaches to the host cell
    4. The cell bursts
    Show the answer

    Answer: The virus attaches to the host cell

    Attachment comes first.

Check you have got it

Answer 6 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.

Practise this lesson free (6 questions)All GCSE and International GCSE Biology lessons

Video for this lesson coming soon. Subscribe on YouTube

Sources and references

Checked by the GCSE-ready editorial team, 16 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

What do exam command words like explain and evaluate mean? →

Related lessons