7 contagious things you didn't know you could catch

By Naveed Saleh, MD, MS
Published June 17, 2020

Key Takeaways

The infectivity of COVID-19 has been on everyone’s minds over the past few months. Research that helps experts better understand its spread keeps rolling in, including a study published in Nature Medicine, which tracked transmissibility and viral shedding in 94 Chinese patients with the virus. The researchers found that 44% of cases were spread when the infected individuals were presymptomatic, and viral load peaked at symptom onset. But infectious diseases are far from the only contagious phenomena in the world. 

In lighter news, here are seven surprising things you didn’t know you could catch.

Chill out, dude!

My, what cold hands you have! Simply empathizing with another person’s exposure to cold temperatures can change the observer’s hand temperature, according to the results of a study published in PLOS One.

Researchers had 36 young adults rate videos that showed actors submerging their right or left hands in warm or cold water, as well as controls with actors placing their hands in front of water without immersion. The researchers then measured temperatures of the observer’s hands. 

Observers first rated videos of actors submerging their hands in cold water as being cooler than those immersed in warm water, thus supporting empathy. Hand temperature dropped in the left hand or right hand when observers observed left-hand or right-hand submersion, respectively, in cold water (no mirroring was observed). No change in hand temperature occurred when observers watched actors submerge either hand in warm water.

Specifically, t-tests, which were used to determine mean differences between submerged hand temperature vs control, demonstrated a 3.54 °F drop in observers’ left-hand temperatures (P = 0.001) and a 2.33 °F drop in their right-hand temperature (P = 0.026). Their hands showed no temperature change (P > 0.1) when they viewed warm-water exposure. “Temperature contagion” was also predicted by observers’ levels of self-reported empathy.

“Mimicking another person is believed to help us create an internal model of their physiological state which we can use to better understand their motivations and how they are feeling,” said the lead author of the study. “Humans are profoundly social creatures and much of humans’ success results from our ability to work together in complex communities—this would be hard to do if we were not able to rapidly empathize with each other and predict one another’s thoughts, feelings, and motivations.”

As for a specific mechanism underlying decreased hand temperatures in observers, the investigators offered a nuanced explanation based on previous research:

“Insight into the mechanism underlying temperature contagion may also be usefully informed by studies of disrupted body ownership induced either experimentally using the rubber hand illusion or in the clinical disorder cold-type complex regional pain syndrome (CRPS). In both of these conditions, unilateral disruption of body ownership is associated with a localised reduction in body temperature suggesting that the conscious sense of our physical self and its physiological regulation are linked.”

Shared waistlines

Obesity has become a big problem in the United States (and elsewhere) over the past 30 years. This phenomenon begs the question: Is obesity “contagious”? In a search for possible answers, researchers mined the Framingham Heart Study for BMI data on participants and their spouses, siblings, friends, and neighbors.

Results from the study, published in the New England Journal of Medicine, indicated that a person’s risk of becoming obese increased by 57% if a friend became obese during a given time period. In adult siblings, the risk was increased by 40%, and with spouses, 37%. No relationship was observed among neighbors. Moreover, persons of the same sex more greatly influenced one another, and smoking cessation did not confound the obesity contagion. 

“Our study suggests that obesity may spread in social networks in a quantifiable and discernable pattern that depends on the nature of social ties. Moreover, social distance appears to be more important than geographic distance within these networks,” the authors concluded. 

While this means that the spread of obesity occurs largely through social networks, this finding also suggests that it may be possible to use social ties to slow or reverse the spread of obesity. 

“Network phenomena might be exploited to spread positive health behaviors, in part because people’s perceptions of their own risk of illness may depend on the people around them,” wrote the authors. 

Although identifying mechanisms for the spread of obesity was outside the scope of the current study, the researchers hypothesized that contagion may have something to do with how people perceive being obese:

“Obesity in alters might influence obesity in egos by diverse psychosocial means, such as changing the ego's norms about the acceptability of being overweight, more directly influencing the ego's behaviors (eg, affecting food consumption), or both.”

Finally, factors like shared diet had nothing to do with obesity contagion, per the authors. “Our findings that the weight gain of immediate neighbors did not affect the chance of weight gain in egos and that geographic distance did not modify the effect for other types of alters (eg, friends or siblings) helps rule out common exposure to local environmental factors as an explanation for our observations,” they wrote. 

Slimmer together

As indicated above, weight loss may also be improved through social ties. For example, researchers from Providence, RI, assessed weight loss in young adults vs older adults after participation in a 12-week, team-based weight-loss campaign. They published their results in Obesity

“Enrollment and weight loss outcomes among [young adults (YA)] in this low‐intensity, team‐based weight loss campaign were quite promising, with over 700 YA completing the program and on average achieving a 4.5% weight loss,” the authors wrote. 

“Furthermore, average weight losses were higher among YA than [older adults]. Indeed, the potential public health impact of such an approach is substantial, suggesting that future efforts to develop weight loss programs for YA may benefit from using a low‐intensity, team‐based approach,” they concluded.

The authors also suggested why this specific intervention worked better for young adults vs older adults. Specifically, it had a lot to do with the way the weight-loss program was framed. They wrote that a low‐intensity, team‐based weight-loss campaign may “have more appeal to this age group than typical clinic‐based weight loss programs. There are a variety of reasons that this type of program may resonate with YA, including the limited time commitment, online system, and the ability to join with their friends as members of a team.” 

If you’re happy, I’m happy

Is happiness infectious? If so, how close do you need to be with another person to infect them with emotion? In a study published in Statistics in Medicine, researchers leveraged observational and experimental datasets from the Framingham Heart Study, the National Longitudinal Study of Adolescent Health, and others to characterize interpersonal influence on a gamut of factors, such as obesity, smoking, cooperation, and happiness.

Previous research has shown that contagion of affective states—including loneliness and happiness—has lower penetrance than health behaviors like obesity and smoking. For the latter, physical distance made no difference in causing contagion. 

“When we turned to studies of affective states (happiness, loneliness, and depression), we found a different result. Associations were only positive for friends and siblings who lived nearby (within a few miles),” the researchers wrote. 

One interpretation of this is that happiness, depression, etc, may require physical proximity to spread. This concurs with other psychological studies on the spread of emotions through face-to-face contact.

But, another interpretation is that these results may be due to context. That is, people in a given neighborhood, exposed to the same environment, might tend to have the same emotional reaction to the same stimuli. To test this possibility, the researchers compared outcomes between next-door neighbors and same-block neighbors (people who live within 100 m of one another).

They found that associations were significant for next-door neighbors, but not for others. “Thus, although it is still possible that contextual effects explain some of the association, they would need to be ‘micro-environmental’ contextual effects that would not affect everyone on the same block,” the researchers explained. 

In other words, infecting someone with happiness requires close proximity. 

Cavities are contagious

File this next contagion under just plain gross. Mothers who clean spoons or pacifiers in their own mouths or with their own saliva can spread cavities to their babies, according to experts.

In a study published in the Indian Journal of Medical Research, investigators examined the transmission of bacterial strains that cause cavities (mutans streptococci) in 30 child-mother pairs via genetic analysis.

The team found that from ages 0 to 6 months, 30% of children were colonized with mutans streptococci. By 30 months, this number was 100% (P < 0.001).

“The present findings showed increasing mean colony counts among children with increasing age,” the authors wrote. “Mode of delivery and habit of kissing by mother did not show significant influence on MS colonization in the children, whereas feeding habit, gum cleaning and number of erupted teeth in children and sharing of the spoon with mother and family members showed significant influence.” 

The clincher: Polymerase chain reaction analysis indicated vertical transmission between mother and child. 

Sharing is caring

Results from observational studies for a gamut of behaviors have demonstrated spread in social networks, which could be due to the tendency of people to befriend like-minded individuals. In a study published in PNAS, researchers set out to examine the spread of cooperative/uncooperative behaviors in strangers playing a public goods game. (Players in a public goods game choose an amount to secretly contribute to a pool for the public good. That amount is multiplied, and the jackpot is distributed equally among players. The more that players contribute, the more they could collectively win. So, cooperation is often, but not always, rewarded over self-interest.) 

“We show that, in both an ordinary public goods game and in a public goods game with punishment, individuals are influenced by fellow group members’ contribution behavior in future interactions with other individuals who were not a party to the initial interaction,” the authors wrote. “Furthermore, this influence persists for multiple periods and spreads up to three degrees of separation (from person to person to person to person).”

What does that mean? The authors explained: “When subjects copy the cooperative behavior of others with whom they interact, their doing so causes them to deviate even more from rational self-interest and may help reinforce this [cooperative] behavior.”

One yawn, two yawns, three yawns, four

Contagious yawning has been noted in not only humans but also dogs, chimpanzees, and monkeys. It can be triggered by seeing another person yawn, but the propensity to do so varies from person to person. Why is yawning more contagious for some people but not for others? 

In a study published in Current Biology, researchers examined the neural basis for yawning using transcranial magnetic stimulation  in 36 adults who were shown videos of actors yawning. First, the researchers found that telling a person not to yawn significantly increased the individual’s urge to yawn.

Second, the researchers found that people with motor cortices that are more easily excitable tend to have contagious yawns more frequently. 

Even so, these two findings taken together accounted for only about 50% of the variability in contagious yawning. What accounts for the remaining 50%? The researchers didn’t speculate, so we apparently still have much more to learn about yawning. 

On the plus side, this study adds to the research on other conditions associated with involuntary movement, such as tics that occur in Tourette syndrome. 

“We suggest that these findings may be particularly important in understanding further the association between motor excitability and the occurrence of echophenomena—observed in a wide range of clinical conditions, eg, epilepsy, dementia, autism, and Tourette syndrome, that have been linked to increased cortical excitability and/or decreased physiological inhibition,” the researchers concluded.

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