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While most of us are familiar with viruses, bacteria, and parasites being contagious, we tend to give a bit of extra space when someone has contracted diseases like Covid-19, ringworm, or lice. Yet, there are numerous other factors that can spread from person to person, some of which can't be defeated by common disinfectants like Clorox wipes or hand sanitizer. For example, contagious behaviors like laughing and yawning are well-known to spread between people.
From unseen bacteria to behaviors we thought we could control, there are countless infections we're unknowingly passing on to one another. Keep reading to discover 10 things you probably didn’t realize could be transmitted from one person to another.
10. Heart Disease
For years, heart disease was thought to be an illness that was not contagious, often attributed to genetics and personal lifestyle choices. However, a recent study by CIFAR’s Humans and Microbiome program has uncovered evidence to challenge that view. Their research revealed that heart disease, along with other conditions like IBS and type 2 diabetes, can be contagious too.
The research team started with the premise that these diseases are linked to a disrupted microbiome, the collection of bacteria, fungi, and parasites found in our gut. When these imbalanced microbiomes are introduced into animal models, they cause disease in the new host. This unhealthy microbiome that leads to these illnesses spreads between individuals, much like how a stomach virus does.
Whether or not a healthy microbiome can be transferred from one person to another remains an area of ongoing study.
9. Loneliness
Ironically, loneliness itself is contagious. Although it may seem that a group of lonely individuals would share common ground and feel less isolated, that's not the case. A long-term study involving over 5,000 participants and their social relationships demonstrated a discernible spread of loneliness from person to person.
People begin to experience loneliness even before they completely withdraw from social circles. They can feel lonely while still being around friends, which is how they transmit their feelings of loneliness to others. These feelings take root in the next person, continuing the cycle until one eventually becomes entirely isolated.
Loneliness is more likely to spread from friends than from family, with women being particularly more affected than men.
8. Ulcers
Stress and spicy foods have often been mistakenly blamed for causing peptic ulcers, but the true culprit is a bacterium called Helicobacter pylori.
Over half of the global population is infected with Helicobacter pylori. While it's unclear why, some people show no symptoms at all, while others develop peptic ulcers in their stomach and small intestine due to the bacterium. This ulcer-causing bacterium is contagious through saliva and fecal matter, meaning that sharing saliva or poor hand hygiene could result in contracting an ulcer.
7. Feeling Cold
Researchers from the University of Sussex have found evidence that humans are vulnerable to temperature contagion, a phenomenon where merely watching someone in a cold environment causes our own body temperature to drop. In a study, participants watched a video of someone submerging their hand in ice water. Not only did they report feeling cold, but the actual temperature of their hands decreased noticeably.
Interestingly, temperature contagion was only linked to feeling cold. When participants watched someone dip their hand into warm water, there was little to no physical response from the viewers.
6. Happiness
We all know from experience that being around cheerful people tends to lift our spirits, while pessimistic individuals can quickly turn a positive event, like winning the lottery, into a downer. It turns out, however, that our happiness is influenced by more than just those closest to us.
While you might not have picked your home based on the moods of your neighbors, their attitudes play a significant role in your well-being. If you have a happy sibling nearby, it increases your chances of happiness by 14%. A joyful neighbor next door boosts your happiness by 34%, while a cheerful friend living a little further away raises your odds of happiness by over 40%.
5. Cancer
There are three animals currently affected by transmissible forms of cancer.
Tasmanian devils transmit facial tumors through bites, dogs spread venereal tumors via sexual contact, and soft-shelled clams pass on cancerous cells through seawater. These discoveries over the past few decades have significantly changed our understanding of cancer's potential to spread as a contagious disease.
While humans are not known to easily pass on cancerous cells from one person to another, there have been rare instances where this has occurred. In 2018, an organ donor who had undiagnosed breast cancer passed away. Four recipients of her organs all developed breast cancer. In another case, an HIV-positive patient contracted cancer from a tapeworm in his intestines. In both instances, the individuals who developed cancer had weakened immune systems. Currently, scientists believe that the transmission of cancer in humans is highly improbable outside of these unusual cases, though the occurrence of transmissible cancer in animals suggests that this could change.
4. Bad Behavior
Most of us understand how being around a group of rule-breakers can lead us to follow suit, even if we wouldn’t have done so under normal circumstances. Research from a series of experiments in the Netherlands shows that this tendency extends beyond just that moment. Not only are we more likely to break rules when surrounded by others doing the same, but we’re also more likely to break other rules in different situations.
If we’re walking alongside a group of litterbugs, we might find ourselves tossing our own trash on the street. Later, when we’re on our own, that exposure to norm-breaking behavior might influence us to sneak some snacks from the office breakroom or forget to clean up after our dog during their walk.
3. Cavities
Among the usual dental advice for maintaining healthy teeth – flossing, brushing, cutting down on sugar – avoiding people with poor dental hygiene probably isn’t on the list.
Cavities develop when bacteria in our mouth convert sugar into acid that sticks to our teeth and wears away at the enamel. Even if you brush and floss regularly, kissing someone with poor oral hygiene will transfer the same cavity-causing bacteria to your mouth.
Children are more vulnerable to cavities because their immune systems haven't developed enough resistance to bacteria. A parent can transfer cavity-causing germs to their child through actions like kissing them on the lips, sharing food utensils, or cleaning a pacifier by putting it in their own mouth before giving it back to the baby.
2. Goals
Through a phenomenon known as goal contagion, we tend to unconsciously adopt the goals of those around us. For example, at a dinner party, if someone is focused on relaxing and having fun, we will usually pick up on this and follow their lead, without them needing to say anything. We end up enjoying ourselves too. However, if we're with someone who's worried about leaving early, we may also start feeling the urge to check the time and wrap up the evening early.
Goal contagion is more powerful when the person influencing us is in our inner circle, compared to a stranger. It is also less likely to happen when the goal seems inappropriate. For instance, if our dinner companion is secretly slipping roofies into drinks, we won’t feel compelled to imitate them.
1. High Blood Pressure
High blood pressure, typically linked to poor dietary and lifestyle habits, has also been found to be connected to a widely spread viral infection.
Researchers found that mice infected with cytomegalovirus (CMV) showed elevated blood pressure compared to those without the virus. Diet still matters—mice with CMV who consumed high-cholesterol foods experienced the highest blood pressure. Further studies with human cell cultures also revealed that CMV-infected human cells produced a protein known to contribute to high blood pressure.
CMV is believed to affect between 60% and 99% of adults globally. The virus spreads through bodily fluids and remains in the body for life.
