The cloud has baffled astronomers since its discovery in 2018. Now, a new study offers a possible cause for its appearance.

When actor Christina Applegate revealed that she had been misdiagnosed with multiple sclerosis, it triggered not only an outpouring of empathy for her, but raised important questions about how often people are told they have the condition when they don't. William Brangham discussed why MS can be so difficult to diagnose with Dr. Michael Kornberg of Johns Hopkins University.

Although the case is considered exceptional, investigators are working to determine how the infection occurred and whether the pathogen played a role in the patient’s death.