We've been hearing a lot about opioid overdose deaths becoming much more common in Minnesota – and the rest of the country – in recent years.

But at a conference this week of health researchers nicknamed the "disease detectives" there was a sobering message based on cases from Minnesota: we're not counting all the opioid deaths because some are being listed as unexplained.

The Centers for Disease Control released the study results collected by a field officer in Minnesota. She looked at a decade's worth of deaths classified as unexplained and found that in 59 of them there was evidence of opioid use.

That's not a ton of additional deaths (Minnesota had already listed more than 2,200 during those 10 years). But the takeaway is this: opioids weaken your immune system so users are more likely to develop diseases like pneumonia that make it harder to sort out what really caused them to die.

'We know we're missing cases'

The research was done by the CDC's Dr. Victoria Hall, who tells CNN: "While my research cannot speak to what percent we are underestimating, we know we are missing cases. It does seem like it is almost an iceberg of an epidemic."

Hall described one of the cases from her research to CNN. A middle-aged man had been taking opioids for back pain and family members worried he had started to abuse them. When he died suddenly, a medical examiner found he tested positive for both pneumonia and a toxic level of opioids. But only the pneumonia was mentioned on the death certificate.

In the conclusion of the study released Monday, the CDC says it's hard to disentangle the effects of opioid use and infectious diseases. But understanding how they interact will be important to preventing more opioid-related deaths.

Learn more about opioids

Opioids are a family of painkilling drugs. Many, like OxyContin, Percocet, and Vicodin are prescribed by doctors. Others, like heroin and fentanyl, usually come from street dealers.

The CDC says opioid overdoses are killing 91 people per day in the U.S.

There's useful information here for people who think they or someone they know might be addicted.

Also, Minnesota recently made an overdose antidote available at drug stores. First responders have used Narcan to reverse the effects of heroin overdoses and health officials suggest regular users of opioids get the anitdote.