With the S&P 500 falling into bear market territory, you might wonder if investing right now is a good idea. It's impossible to predict when the market will hit bottom and then rebound. But one thing is for sure: After tough times, the market has always bounced back. So that means the best thing to do during a market downturn is actually to invest. You'll reap the rewards down the road.

Here, I'll talk about three healthcare stocks that are incredibly cheap right now. They've each posted double-digit declines since the start of the year. Let's take a closer look at why they're smart stocks to buy.

1. Abbott Laboratories

What I like most about Abbott Laboratories (ABT -1.19%) is its diversification. The company has four solid businesses: Medical devices, diagnostics, pharmaceuticals, and nutrition. Traditionally, the medical devices business has contributed most to revenue. At certain stages of the pandemic, though, device sales dipped as hospitals postponed procedures. At the same time, Abbott's vast array of coronavirus tests drove gains in the diagnostics business.

As the pandemic eases, coronavirus testing sales may slip -- but that's OK. Fewer coronavirus cases in hospitals results in hospitals doing business as usual. And that means filling up the calendar with surgeries. In that scenario, we can expect Abbott's medical device sales to strengthen. So, Abbott is able to deliver solid revenue whether the health crisis persists or the situation returns to normal.

A look at Abbott's financial situation shows earnings as well as return on invested capital and free cash flow have generally been on the rise over the past few years.

Chart showing Abbott's net income rising, revenue staying level, and free cash flow and return on invested capital on upward trend since 2018.

ABT Net Income (Annual) data by YCharts

And since the beginning of the year, while valuation has fallen, revenue continues to rise. All of this means Abbott shares look cheap considering today's picture and future prospects.

2. Pfizer

Pfizer (PFE -1.53%) not only dominates the coronavirus vaccine market -- it is also the leader in the treatment market. The company sells the Comirnaty vaccine, and the coronavirus treatment pill Paxlovid. Comirnaty brought in more than $36 billion in sales last year. And the company predicts $32 billion in Comirnaty sales and $22 billion in Paxlovid sales for this year.

Of course, once the pandemic is over, these levels may decline. But they still may remain significant. Here's why. Experts say the coronavirus will stick around. And we've seen that the virus has mutated, resulting in new variants. Pfizer aims to update its vaccine/booster as needed to address this. It could become part of many people's fall vaccination ritual, just like a flu vaccine.

Pfizer is trading at only seven times forward earnings estimates -- down from more than 12 at the start of the year. Some might say that drop makes sense, considering coronavirus program revenue may slip.

Importantly, though, Pfizer doesn't depend on the coronavirus vaccine. The company has eight other blockbuster products. Some of these are set to face generic competition by the end of the decade. But Pfizer has a good chance of compensating for that over the long term.

That's because it has 29 candidates in late-stage clinical development. At least some of them could represent revenue in the not-too-distant future. And that means Pfizer's price today looks like a deal.

3. Teladoc Health

Teladoc Health's (TDOC -3.59%) business soared during the early stages of the pandemic. People opted for virtual medical visits rather than visits to doctors' offices. But this wasn't just a short-term trend. Revenue and medical visits already were climbing at Teladoc prior to the pandemic. Those metrics continue to climb now in the double digits -- when people are no longer isolating.

Teladoc offers a broad range of services that address the "whole person" -- from physical to mental health. It's gaining in membership numbers and spend per member. These are both key in the company's ability to grow revenue. Teladoc predicts about 20% revenue growth for this year.

Why have Teladoc shares dropped? Some investors worry because the company isn't yet profitable. But the lack of profitability isn't surprising at this point in Teladoc's story. Shares fell further when the company posted an enormous non-cash goodwill impairment charge in the first quarter. That was linked to its purchase of Livongo.

Yes, Teladoc paid too much for Livongo. But Livongo's chronic care portfolio could represent an important part of the picture over the long term. That's because chronic care represents a major growth area -- and Teladoc hasn't yet finished integrating Livongo products.

Today, Teladoc trades for only two times forward sales estimates. That's down from more than six at the start of the year. This looks cheap considering that Teladoc is still in the early stages of its growth story.