On September 18 Orianna Carvalho woke up at 3 am with a sore throat and the sniffles. At first she thought her symptoms were caused by allergies. But as the minutes ticked by, she began to worry they were caused by Covid-19. The following morning, Carvalho got tested at the University of Rhode Island, where she is a first-year doctoral student. Over the next few hours she developed a fever, and the catastrophizing began in earnest. When Carvalho finally learned that the cause of her misery was not COVID but the common cold, she was relieved but also surprised. “I have been so careful—wearing a mask every time I go somewhere, keeping at least six feet away from other people, using hand sanitizer and washing my hands,” she says. “I don’t know how I got sick.”

Carvalho is not alone. Many Americans have been puzzled to find that their best efforts to avoid Covid-19 have not always protected them from less troubling infections such as colds, stomach bugs and strep throat. How have other pathogens slipped through our anti-COVID defenses? There are no clear-cut answers, but the work of infectious disease specialists, virologists and epidemiologists—much of it conducted decades before the current pandemic—provides some clues. Their research shows that many microbes are more numerous, hardy and contagious than SARS-CoV-2, the virus that causes Covid-19. And for many of us, even our best efforts are not good enough.

The public health measures taken to stem the spread of SARS-CoV-2, which has been responsible for the deaths of more than 207,000 people in the US to date, have also affected the prevalence of other respiratory viruses. This year the Southern Hemisphere essentially skipped flu season, which typically hits countries such as Australia, Chile and South Africa in May or June. Data from Australia suggest that although pandemic restrictions pushed many non-flu viruses out of circulation, a group of cold-causing pathogens known as rhinoviruses stuck around. A similar trend could be in store for the US, according to researchers who are tracking transmission of respiratory viruses in New York State, Washington State and Texas. Pedro Piedra, a pediatric infectious disease specialist at Baylor College of Medicine, says that although he has seen a significant decrease in many common respiratory viruses during the pandemic, he has noticed an uptick in rhinoviruses this fall.

Some virologists believe that the sheer number of viruses that cause the common cold can make it exceedingly difficult to avoid catching one: there are around 200 different pathogens. These include four coronaviruses (the group that includes SARS-CoV-2); four parainfluenza viruses (which, despite their name, bear no relation to influenza viruses); respiratory syncytial virus; and 160 different rhinoviruses. Viral censuses have revealed that dozens of these rhinoviruses circulate in any one place at a given time. “You might be immune to the flu, but you are not going to be immune to all those rhinoviruses,” says James Gern, a rhinovirus researcher at the University of Wisconsin–Madison. “That’s one unique feature of rhinoviruses—you are always going to be susceptible to some.”

But there is only one SARS-CoV-2 virus, and it has proved to be more than enough to wreak havoc on our lives. The persistence of rhinoviruses during the pandemic may be the result of not only their impressive number but also their primitive nature, says Ian Mackay, a virologist at the University of Queensland in Australia. Similar to the flu virus, SARS-CoV-2 is a more highly evolved virus that is enclosed in a fatty “lipid” membrane. This envelope can cloak the pathogen from antibodies deployed by the human immune system, enabling it to infect cells undetected. But it can also break down after exposure to the environment or a good handwashing, rendering the virus harmless. Rhinoviruses, on the other hand, never evolved an envelope. These so-called naked viruses, which also include the gut-distress-inducing noroviruses, are more resistant to sanitizers and disinfectants and may last longer on fingertips and surfaces.

Although it is possible to pick up respiratory viruses from contaminated surfaces, most experts say we are more likely to get sick through contact with infected people. In 1969 half of a group of men wintering at a remote Antarctic base developed signs and symptoms of the common cold after being isolated for 17 weeks. Scientists never identified the source of the outbreak, but Mackay and others think it is possible that the men entering the base might not have been as healthy as they looked. Asymptomatic spread has gotten a lot of attention during the Covid-19 pandemic: studies suggest 40 to 45 percent of SARS-CoV-2 transmission comes from people not yet showing symptoms. Many colds and flus may also be passed along by people who do not have symptoms, although to what extent this spread occurs is an open question. At least one study detected rhinoviruses in a third of asymptomatic children.

“Children, in particular, are a petri dish for transmission,” says Arnold Monto, an epidemiologist at the University of Michigan, who studies the spread of respiratory illnesses within households. Because kids are prone to eye rubbing and nose picking, they can quickly contaminate their home with a menagerie of viruses and bacteria. Unlike the acute respiratory infections that typically come and go in a matter of weeks, children can harbor chronic infections with bacteria—such as Streptococcus pyogenes, which causes strep throat—for months before ever making themselves or others sick. Tara Smith, an epidemiologist at Kent State University, says it is unclear how such bacteria move from harmless colonizer to invasive pathogen, but the stress of the pandemic could play a role. And kids are not the only germ factories in our homes: pets are common carriers of many pathogens. “People probably get sick from their animals more than we realize,” Smith says.

Despite the myriad possibilities, many experts believe the explanation for why some of us are still getting routine infections is fairly mundane. “Some people may think they are better protected than they actually are,” Smith says. Gern agrees: “If cold viruses are still spreading, that means we are still having person-to-person contact,” he says. We live in a world where once beneficial actions—such as hugging a friend or going to the gym—now pose heightened risks to our health. For her part, Carvalho thought she was doing everything she could to be safe. After months of staying home, she returned to the gym for some socially distanced martial arts. She now suspects that it is how she got sick.

Since the beginning of the pandemic, more than 80,000 people who wondered if they had a COVID infection have called the telemedicine company Doctor on Demand, according to Prentiss Taylor, a physician and the company’s vice president of medical affairs. More than half of those cases were not referred for COVID-19 testing because some other respiratory affliction was deemed more likely. Under the circumstances, catching a cold instead of COVID might feel like dodging a bullet. But the fact that other viruses have been able to slip through our defenses could serve as a warning for future pandemics, Mackay says. “If we ever see a new rhinovirus come along, we will have even more trouble containing it than SARS-CoV-2. A rhinovirus pandemic would be a massive threat that would spread like that,” he adds, snapping his fingers. “And there’s no guarantee it would only cause common colds.”