As the Zika virus outbreak continues to worsen, we keep getting more bad news on this mysterious virus: now, researchers have linked the Zika infection with a rare joint birth defect, adding to the concerns about the effects of the virus, accoring to a BMJ statement.

Researchers determined that a small number of babies who are infected with Zika develop a rare birth defect that causes muscle weakness and stiff joints. This condition, called arthrogryposis, is just another one of the set of problems that a Zika infection brings to a pregnant mother — it’s already been linked to microcephaly, which causes an abnormally small brain and head in a newborn child. Scientists have also blamed the virus for eye problems, hearing loss and stunted growth.

Scientists are even referring to the entire package of possible health problems linked to a Zika virus infection as congenital Zika syndrome, and they are still trying to wrap their heads around what else the syndrome might involve.

The scientists based their conclusions on analyzing information from seven babies with anthrogryposis in Brazil. The condition is rare, affecting just one out of 3,000 live births in the United States. Six of those seven babies also had microcephaly, and all had signs of infection in the womb.