When people get the flu they might infect 1.28 other people. How can you infect 1.28 people? This is an average number taken from studies that track how flu spreads between people.

People with Covid 19 can infect about 3 other people. Scientists think people with the Delta variant could infect another 7 people. This number is called the “R” number - the “reproductive factor”. A higher R number means a disease will lead to more illnesses, and might overwhelm health care systems like the NHS.

The flu is caused by lots of different viruses that were once a lot more dangerous. Over hundreds of years human immune systems have learned how to recognise and fight these viruses. Once your body recognises an illness, it can fight it more effectively, even if the virus adapts.

Covid-19 is a new illness. Our immune systems don’t recognise it, and find it very hard to fight off at first. This is one reason it’s making so many people so sick.

This section has been fact checked with:

• GAVI Gavi, the Vaccine Alliance helps vaccinate almost half the world’s children against deadly and debilitating infectious diseases.

• The Guardian a reputable news and media organisation