Antibiotics and painkillers: It is difficult to develop drugs that kill viruses without also damaging the body’s tissues.

The idea

Viruses reproduce inside host cells, so a drug that kills the virus must either enter the host cell or interfere with viral replication without harming the host cell. Because viruses use the host cell's machinery, many antiviral targets are also present in human cells, making it hard to avoid damage to body tissues. This is why antibiotics do not work on viruses and why developing antiviral drugs is challenging. For example, a drug that blocks viral protein synthesis might also block human protein synthesis, causing side effects. Antiviral drugs therefore tend to target virus-specific enzymes or structures, but even then side effects can occur. The immune system usually clears viruses, and vaccines help prevent viral infections.

What earns the marks

• Viruses reproduce inside host cells, so drugs must affect the virus without harming the host cell.
• Many antiviral targets are similar to human cell processes, so damage to body tissues is a risk.
• Antibiotics do not kill viruses, which is why viral infections are not treated with antibiotics.
• Antiviral drugs are difficult to develop because they must be selective for virus-specific features.
• The immune system and vaccines are important in defence against viral infections.
• A named example, such as a drug that blocks a viral enzyme, can show how selectivity is attempted.

Common mistakes and how to put them right

• Thinking antibiotics can treat viral infections.

  Put it right: Antibiotics kill bacteria, not viruses, and do not work on viral diseases such as colds or flu.

• Assuming antiviral drugs are harmless because they target viruses.

  Put it right: They can still damage body tissues because viruses use host cell processes.

• Believing viruses are easily killed outside cells.

  Put it right: Viruses replicate inside host cells, so drugs must act within the body without harming host cells.

Exam tips

• Explain that viruses reproduce inside host cells and link this to the difficulty of selective drug action.
• State that antibiotics are ineffective against viruses to contrast with the challenge of antiviral drug development.
• Use the phrase 'without damaging the body's tissues' to address the second clause of the statement.