Pandemics past and present have been caused when pathogens – germs that cause disease – move between animals and humans, as SARS-CoV-2 (the virus that causes COVID-19) did when it made its way from bats to people. But not all emerging pathogens have it so easy.
Satellites are a peculiar group of tiny pathogens that hijack unrelated viruses (called their “helpers”) to spread. Satellites are notorious because they can make the disease caused by their helper viruses more severe.
One example is the hepatitis delta virus, better known as hepatitis D or HDV, which is the only satellite known to cause illness in humans.