You can t predict with any accuracy the future health of any one individual because there are just too many random events that can occur to an individual. But you can predict with reasonable certainty the health needs of 500 people or even 100. Across a broad group the random events tend to cancel each other out. This is not to say it is easy to predict Index funds. But it does highlight that once a trend has begun, it is much less likely to be reversed by a random event than an individual stock is.
Index funds have a number of advantages over individual stocks.
Perhaps the biggest are the power of leverage, which gives the index larger daily moves, requires less capital, and is easier than buying and selling individual stocks. Many stock market investors can attest to the difficulty in picking the right individual stocks at the right time. Few have that kind of time or resources. If they do find a stock they like, it may not respond as they expect for a number of reasons. Even if earnings meet expectations, the price of the stock may fall, for example, because the market was disappointed it didn t perform better.