 The fight against coronavirus means that everyone is talking about vaccines. But how do vaccines work? And what have ferrets got to do with it? Many human diseases, like COVID-19, are caused by tiny particles called viruses. And by tiny, we mean really, really tiny, much smaller than a human self. A lot of viruses have spikes on the outside that are made of protein. The protein spikes help the virus to attach to a cell and eventually break into the cell where it can do the real damage. This is what makes us ill. Luckily, when a virus manages to get into our bodies, we are not less defenseless. Special cells circulate in the bloodstream and hunt down viruses and other things that can make us ill. These cells are part of the immune system. Two of the most important are called phagocytes and B cells. Phagocytes destroy other cells. The name actually means cell eater. B cells make the protective antibodies that kill particular viruses and other diseases. When viruses get into you, the phagocytes seek them out and try to engulf them. When the virus particle is completely engulfed by the phagocytes, it is broken up and destroyed. But the phagocytes doesn't throw away all the bits of the virus straight away. It keeps the viral protein spikes. The thing the virus uses to attach to your cells and it shows them to the B cells. The B cells can now start to make antibodies which are a perfect fit for that particular protein spike. Millions of these antibodies are released into the blood. The antibodies stick to the virus so it cannot attach to your cells any longer. Once B cells have made antibodies for a particular virus, they will usually remember it for a long time. If the virus comes back, the B cells make more antibodies straight away before it can hurt you. This is called immunity. Vaccines work by teaching your body to make antibodies without having to meet the real virus. So scientists take the virus or one very similar to it and remove the parts that do damage, which is usually the genetic material or DNA. What is left is a depleted virus, something that cannot hurt you but which still triggers the B cells. But how do you make sure that the depleted virus really can't hurt us? That's where ferrets come in. Vaccines have to be tested on something that has an immune system to see if it is safe and effective. Ferrets are useful for studying flu-like viruses because they catch colds like humans and have similar symptoms. But many other animals are essential in the hunt for new vaccines, especially the laboratory mouse, which is the most commonly used animal in science by far. Even chickens help save lives. The viruses we study are often grown in chicken eggs, another essential part played by animals in vaccine research. In countries all over the world, animal studies are an important part of the search for new vaccines for coronavirus and other conditions. This benefits people and animals everywhere.