The first diagnosis of an Ebola case inside the U.S. added pressure to an already shaky stock market and helped spur a flight to safety in Treasurys, as the deadly virus became the latest macro concern to jar markets obsessed with a decline in global growth.

But traders say the selling was only partly due to macro concerns, and there were some signs of investors shifting out of stocks and buying bonds, despite the fact that stocks are often bought at the start of the quarter.