Viruses.

Step 1 of 7

Not a cell

Pathogens are tiny organisms and particles that get into the body and multiply there: bacteria, viruses, fungi and parasites. A bacterium is a whole living cell, which divides by itself. A virus is something else entirely.

Cut open is at Cut open: the coronavirus is opened from the front, and its genome lifted out. Look at what is inside it, and what is not.

Step 2 of 7

How small

Most viruses are between twenty and three hundred nanometres across, millionths of a millimetre. An ordinary light microscope cannot make out anything much smaller than 200 nm, so nobody saw a virus until electron microscopes arrived. Switch between the viruses in Virus and read the size of each.

Step 3 of 7

Taking over a cell

A virus drifts until it meets a cell with a molecule on its surface that fits the virus’s proteins. That fit decides which cells it can infect: the coronavirus grips cells lining the airways. Then its genes get inside. Virus and Infection are now fixed at the start.

Step 4 of 7

Genes in a box

Every virus has a genome, DNA or RNA, inside a capsid, a shell built of many copies of the same few proteins. Cut open is at Cut open. In most viruses the genome is one long molecule, but not in all.

Step 5 of 7

Envelopes

Some viruses also wear an envelope over the capsid: a piece of fatty membrane taken from the last cell they left, studded with proteins of their own. Switch between the four viruses in the Scene and compare.

Step 6 of 7

Why not antibiotics

An antibiotic is a drug that kills bacteria or stops them, without harming our own cells. It can do that because it aims at structures only bacteria have. In the Scene, an influenza virus in the middle of copying: who is doing the work inside the cell?

Step 7 of 7

Bursting out

At the end of the cycle the new particles leave the cell: enveloped viruses by budding, and phages such as T4 by bursting the cell. Virus is fixed at Phage T4, which infects bacteria. The Scene shows four offspring; in reality a hundred or more come out.