Communicable (infectious) diseases: Bacteria and viruses may reproduce rapidly inside the body.

The idea

Pathogens cause illness only if they can multiply within the host. Bacteria are single-celled prokaryotes that divide by binary fission: one cell splits into two, so numbers rise geometrically, for example 1 → 2 → 4 → 8, doubling each generation. Viruses are much smaller and are not cells; they must enter a host cell and use its machinery to make many identical copies of their genetic material and protein coat, then burst out to infect more cells. Because both processes are fast and the body is warm and nutrient-rich, a small initial dose can become a huge population quickly, producing symptoms before the immune system controls the infection.

What earns the marks

• Bacteria reproduce by binary fission, in which one parent cell divides into two genetically identical daughter cells.
• Bacterial population growth is rapid and can be exponential when conditions inside the body are favourable.
• Viruses do not divide by binary fission; they replicate only inside a host cell using the host's enzymes and ribosomes.
• Viral replication involves copying the viral genetic material and making viral protein coats, then releasing new virus particles.
• Rapid reproduction inside the body means pathogen numbers can rise before the immune system removes them, so symptoms develop.
• The body provides warmth, moisture and nutrients that support fast pathogen reproduction.

Common mistakes and how to put them right

• Saying viruses reproduce by binary fission like bacteria.

  Put it right: Viruses replicate inside host cells and cannot reproduce independently.

• Describing bacteria as 'growing bigger' rather than increasing in number.

  Put it right: Reproduction increases cell number by division.

• Assuming all bacteria are harmful.

  Put it right: Many bacteria are harmless or useful, and only some are pathogens.

Exam tips

• Use the term binary fission when describing bacterial reproduction, and state that it produces identical cells.
• For viruses, always mention a host cell, because replication depends on the host's machinery.
• Link rapid reproduction to symptom timing: more pathogens means more damage and a greater chance of feeling ill.