Redondoviruses are associated with certain ailments, but whether they cause disease is unclear.

Healthy human lungs are home to a family of 19 newfound viruses — which are present at higher levels in the lungs of critically ill people.

Earth is swarming with viruses, and so far scientists have documented only a fraction of them. But advances in DNA-sequencing technology are allowing researchers to characterize more of these microbes.

Frederic Bushman, Ronald Collman and their colleagues at the University of Pennsylvania in Philadelphia sequenced genetic material in fluid collected from the lungs of human donors and identified two viruses that have circular genomes. The team then screened thousands of previously collected samples — including both human tissue and other materials, such as soil — for similar genetic sequences.

The screen picked up the two new viruses, as well as 17 related viruses. The researchers dubbed the viral family Redondoviridae, after the Spanish word for round.

The team found redondoviruses along the human respiratory tract in both healthy people and patients in a hospital intensive-care unit. Some people with gum disease also had elevated levels of redondoviruses, but the viruses are unlikely to be the cause of the gum inflammation, the authors state.