Time-lapse photography allows us to observe changes in nature that we’d otherwise be unable to see. This photo shows the paths of stars as they circle around the sky over an entire night.

The first step in attenuation is to encourage the virus to infect a new type of cell. As the virus changes and adapts, it gets better at infecting the new cell type and is less suited to infecting the type of cell it previously sought.

Genetic engineering can use bacterial or yeast cells to make proteins. The hepatitis B vaccine is made using genetically modified yeast cells.

Antibodies meant to stop the virus from infecting liver cells are neutralized by excess hepatitis B surface proteins floating in the blood.

A medieval castle was built to defend against any attack. How is the immune system similar to a castle?