The Dow Jones Industrial Average plunged on Wednesday after falling in each of the previous nine sessions, capping its longest daily loss since 1974.

The blue-chip index has slumped 6% during its recent losing streak. The S&P 500 and Nasdaq Composite lost a comparatively modest 3.5% and 1.8%, respectively, although they experienced more painful declines on Wednesday.

Why have these three major indexes, each containing the stocks of some of America’s largest companies, diverged recently?

UnitedHealth’s decline after CEO killing weighs on Dow

Shares of UnitedHealth Group (UNH) lost more than 20% of their value in the nine sessions following Dec. 4, the day the CEO of its insurance unit was fatally shot in an attack that brought renewed focus on the health care business in America directed. That was also the last day the Dow closed higher.

Since the shooting, lawmakers have passed legislation to force the breakup of certain large health care companies, and President-elect Donald Trump has vowed to “cut out the middleman,” referring to pharmacy benefit managers (PBMs) that set drug prices on behalf of insurers negotiate , employers and government health plans.

Shares of CVS Health (CVS) and Cigna (CI), which like UnitedHealth operate insurance companies and PBMs, fell 24% and 20%, respectively, between Dec. 4 and Tuesday’s close.

Before the plunge, UnitedHealth had a stock price of $610, making it the most expensive stock in the Dow and therefore the most influential component of the index. That’s because the Dow is price-weighted. This means that the higher a company’s share price, the greater its influence on the index. The S&P 500 and Nasdaq, on the other hand, are capitalization-weighted, giving more influence to companies with higher market value.

Nvidia’s inclusion in the Dow also causes the index to fall

UnitedHealth wasn’t the only laggard in the Dow this month. Nvidia (NVDA), which joined the blue-chip index in early November, has fallen in all but one trading session since December 4. The stock may have been weighed down by reports of an anti-monopoly investigation in China and the rise of rival Broadcom (AVGO) making a technical correction earlier this week.

The Dow was on track to end its losing streak on Wednesday before Federal Reserve interest rate forecasts sparked a broad selloff. Nvidia shares fell 1.1% on Wednesday after rising more than 3% in midday trading. UnitedHealth Group was the only component of the 30-stock Dow to close in the green, rising 2.9% along with rivals CVS and Cigna.

Update – December 18, 2024: This article has been updated to reflect stock prices and index levels as of market close on Wednesday.