AMNA NAWAZ: Federal health officials are still trying to determine the source of a multistate outbreak of cyclospora infections likely linked to shred iceberg lettuce.

There may be more than 7,000 cases of cyclosporiasis nationwide, the largest recorded outbreak in U.S.

history.

The Centers for Disease Control says more than 1,600 people have been sickened, and another 5,000 in more than 30 states are under investigation.

Our William Brangham has more on the latest.

WILLIAM BRANGHAM: Amna, the FDA said today that a lettuce sample from the company Taylor Farms that was initially reported as positive for cyclospora was actually a false positive.

But the agency also said that, based on other data, it still believes Taylor Farms lettuce is linked to this outbreak.

Some of that lettuce ended up at Taco Bell restaurants.

It is all prompting more questions and adding to the confusion for the public.

So, as we often do when things are confusing in the public health world, we turn to Katelyn Jetelina.

She's adjunct professor at the Yale School of Public Health and the author of the Substack column Your Local Epidemiologist.

Katelyn, great to have you back.

Help us understand the strange back-and-forth that's going on here.

The FDA initially said to Taylor Farms, this is your lettuce, we found a sample of it, and then said, well, no, it's actually not that sample, but we still think it is connected to you.

What do we know concretely about the origins of this outbreak?

KATELYN JETELINA, Your Local Epidemiologist Substack: Yes, so this is a nasty parasite, and the only way to determine the common culprit is through patient interviews followed by lab genetic testing.

And what we're seeing constantly as a very strong signal in patient interviews hasn't changed, that it is some sort of lettuce.

And the FDA traced back data through invoices and shipping records and receipts, narrowed it down to Taylor Farms in Mexico.

Taylor -- the FDA -- regardless of this false positive, the FDA continues to say Taylor Farms is overwhelmingly linked to the outbreak.

And this is for two reasons.

One is because the false positive was on lettuce coming from Mexico that was not part of the recall.

So if it really, truly was a positive, we would need a lot -- much larger recall.

And, second, outbreak investigations don't rely on one single lab test.

They rely on the weight of all of the evidence and, like I said, epidemiological evidence, trace-back data.

And not finding a parasite on one lab sample doesn't mean it isn't there.

So, sure, this false positive whiplash was maybe a small setback, but it did not reverse what scientists have been discovering the past few weeks leading up to this.

WILLIAM BRANGHAM: Do you have a sense as to why this is so complicated?

It always seems like, in past outbreaks, it's very quickly identified, salmonella from this farm on this type of produce.

Why is this one proving to be such a tricky nut to crack?

KATELYN JETELINA: Yes, so this is one of the most opaque, confusing, unclearly communicated outbreak responses I have ever seen.

And it's really the perfect storm of three factors.

One is the parasite itself.

It's nasty.

Symptoms can take up to 12 days to appear.

So patients are being asked to recall meals from like two weeks ago.

And I don't remember the last meal I ate two weeks ago.

So it's a lot of laborious work.

Second is that we have a hollowed-out federal response, right?

Neither the FDA nor the CDC has a permanent director right now.

We have lost a lot of staff, some lab capacity, as well as a lot of reorganization these past 18 months.

And this federal coordinating, as well as centralized communication, is -- or lack of centralized communication is causing a ton of confusion.

And then the third factor we can't ignore is that this is a private company, Taylor Farms, linked to the outbreak really muddling a lot of waters with its P.R.

approach.

So, put together, many of us on the outside are putting these pieces together like a puzzle and trying to read between the lines.

But the most obvious bottom line here is, let's avoid iceberg lettuce in these 27 states until further notice.

WILLIAM BRANGHAM: And anything else beyond that?

I know a lot of people saw the news over the weekend at the end of last week, thinking, OK, I will just avoid Taco Bell and shredded lettuce.

Is there anything else people ought to be doing to protect themselves?

KATELYN JETELINA: Yes, so distribution networks are complex and suppliers don't just serve one fast-food chain.

So this is not just Taco Bell.

This may be grocery stores and food services operations in bulk.

And we don't actually know exactly what is recalled.

We actually have to decode it ourselves.

Taylor Farms supplied a very confusing recall notice.

So the safe thing right now is to avoid iceberg lettuce altogether in these 27 states.

That's the safest thing to do.

If you're high risk, like a pregnant woman or 65 and older or immunocompromised, if you're especially in Michigan or some of these high-case states, I would do extra caution with any produce that has hard-to-watch wash crevices like raspberries.

A third sort of thing you can do is, farmer's markets or local farmers often sidestep these complex bagged lettuce supply chains, which is a partial fix and not risk-free, but it's something that you can do.

This all ends when this contaminated product is out of the supply chain, and that's going to take time.

WILLIAM BRANGHAM: Katelyn Jetelina, always great to hear from you.

Thank you so much for being here.

KATELYN JETELINA: Yes, thank you for having me.