This is the second installation in a series about respiratory viruses and their complexities. The first essay explained less common viruses like human metapneumovirus and adenovirus. Today, we're taking a deeper look at disease surveillance.

Why don’t I know what I have?

If you go to the doctor or hospital with what feels like a “cold,” there’s a set of decisions happening behind the scenes about whether to test you, and for what.

In most outpatient settings, testing is usually “targeted.” This means you might get a single test (for example, just for influenza), or a small “triplex” panel that checks for the most common viruses: SARS-CoV-2, flu, and RSV. These are the pathogens where a diagnosis can directly affect care or public health decisions.

Testing

Force of Infection is now on the summer schedule, meaning national Outbreak Outlook updates are only available to paid subscribers. The winter schedule, including the free national version, will resume in October. To continue receiving reports during the summer, please upgrade to paid.

Influenza-like illness

Flu season officially ended last week, and activity levels continue to decline.

A new Covid-19 variant has been making headlines lately, and this one has an unusual name. BA.3.2, nicknamed "cicada", has been detected in at least 25 U.S. states and 23 countries. Here's what we know so far.

Why "cicada"?

Cicadas are famous for spending years underground before emerging in enormous numbers, sometimes after 13 or 17 years of dormancy. BA.3.2 follows a loosely similar pattern: it was first identified in South Africa in November 2024, circulated at very low levels for over a year largely under the radar, and then emerged more forcefully in Europe last fall before spreading broadly this winter.

Force of Infection is now on the summer schedule, meaning national Outbreak Outlook updates are only available to paid subscribers. The winter schedule, including the free national version, will resume in October.

Respiratory Diseases

Influenza-like illness

We have officially passed out of flu season. Visits to the doctor for flu-like illness dropped to 2.9%, below the baseline of 3.1%.

Force is growing, and so is the team. I'm excited to welcome Gage Moreno, PhD, as a contributing writer. Like our upcoming move to a standalone website, expanding the team is part of the plan to make this publication more valuable to you over the long term. This is the first explainer in a two-part series; Part II will cover how epidemiologists track these pathogens, and what future technologies might improve surveillance. -Caitlin

If you’ve ever been sick with “a cold,” yet tested negative for flu and Covid-19, you’ve already bumped up against a hidden world of viruses.

“A cold” isn’t a single illness it’s a label used for dozens of respiratory pathogens that circulate every year, most of which are rarely tested for. Even though they are largely invisible to patients, these viruses cause real illness, drive outbreaks, and shape seasonal patterns.

What

Yesterday, the U.S. CDC issued a Health Alert Network (HAN) advisory about New World screwworm cases on the Mexico side of the Texas-Mexico border. New World screwworm is primarily a disease of livestock, though humans can become infected. There are currently no human or animal cases in the United States, though it is an encroaching threat.

Against the backdrop of resurgent pertussis and measles, a recent infant botulism outbreak, and a changing vaccine schedule, screwworm feels like a distant concern. Yet clinicians have not received a HAN on these more pressing issues. The last advisory covering a disease with U.S. cases was in March, three HANs ago.

This is the gap I want to close with FOI Clinical, a new outbreak monitoring service I’m launching. There is no quick, easy way for front-line clinicians like pediatricians, family doctors, or emergency physicians to get a recurring, plain-language briefing