Are viruses alive? Why scientists disagree

0:00Are viruses alive? Let's compare a virus with a bacterium, and see why scientists disagree.

0:08Drawn to the same scale, a bacterium is about 1 to 5 micrometres long.

0:14A micrometre is a thousandth of a millimetre, and a nanometre is a thousandth of a micrometre.

0:21Let's zoom in on the virus. Most viruses are only 20 to 300 nanometres across.

0:28Up close, a virus is not just a tiny bacterium. It is built very differently.

0:35At its simplest, a virus is genetic material inside a protein coat.

0:41To decide, scientists check the characteristics of life, the features that living things share.

0:48A bacterium is a single living cell. A virus is not a cell at all.

0:54A bacterium takes in nutrients, the substances it needs for energy and growth. A virus takes in none.

1:03A bacterium usually copies itself by splitting in two. A virus can copy itself only inside a host cell, a living cell that it gets into and takes over.

1:15Yet both have genes: instructions carried in their genetic material.

1:20Both also evolve, which means they change over time as their genes change.

1:26So some scientists say a virus is not alive: it is not a cell, takes in no nutrients, and cannot copy itself on its own.

1:35Other scientists say a virus is alive, because it has genes and changes over time, like living things do.

1:43Who is right depends on how we define life, so the debate goes on.

1:49A virus is not a cell, yet it has genes and changes over time, so scientists disagree on whether it is alive.