17 Surprising Conditions You Didn't Know Were Contagious

Anyone who's ever caught the flu from a coworker or come down with a cold after standing next to someone sniffly on a crowded train can attest to the fact that humans are walking disease vectors. However, while you may be aware of some of the more common ailments you can get from close contact with others, there are a surprising number of other conditions—even potentially fatal ones—you might not realize are contagious. Before you unintentionally put yourself in harm's way, make sure you know these surprising conditions that can be passed on from person to person.

Believe it or not, plantar warts are caused by human papillomavirus, more commonly known as HPV. However, the strain of HPV that causes plantar warts isn't the same one that you probably associate with intercourse.

"While HPV is more 
commonly known for causing cervical cancer, plantar warts are caused by 
different strains of this virus (HPV 1, 2, and 4)," explains Nicole Glynn, MD, a board-certified pediatrician with Getzwell Pediatrics in San Francisco. "The strains of HPV that 
cause plantar warts are not only extremely contagious, [but] they can be transmitted 
from direct contact with another person's wart or by walking barefoot in 
moist environments like showers or around pools."

Yes, that fungus making your nails look funky is highly contagious. According to the American Academy of Dermatology, it's especially transmissible in warm, damp environments. Plus, sharing your shoes with someone when you have an infection can also cause it to spread.

If and when you have fungus on your feet, make sure to avoid walking around barefoot—and generally, you should be extra careful about wearing shower shoes at the gym and double-checking your friends' feet before you share shoes with them.

That sinus infection plaguing you could have been passed on by someone else. According to the Asthma and Allergy Foundation of America, most cases of sinusitis are caused by viruses, so hanging out with a sick individual or even just touching a contaminated surface could result in the uncomfortable infection.

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"Scabies is extremely communicable," says G. Thomas Ruiz, MD, OB/GYN, lead gynecologist at MemorialCare Orange Coast Medical Center in Fountain Valley, California. He notes that it can be spread through person-to-person contact as well as via contaminated surfaces and shared linens and garments.

The small parasite that causes scabies will typically live behind the kneecaps, in the crook of the elbow, and between the fingers in the webbing. Fortunately, treatment isn't particularly onerous: a scabies-fighting body lotion—as well as throwing everything the infected person has worn in the wash—should do the trick.

If you develop a poison ivy rash after doing some weeding in your yard, it's important to limit your physical contact with others and be diligent about wiping down shared surfaces until you've showered thoroughly and washed your clothes. That's because, as Ruiz explains, "poison ivy is an oil, so if someone has an outbreak of poison ivy and someone else touches the rash, the oil transfers over."

If you ever notice a round rash on your skin, it's a good idea to hold off on hugging your friends and family—or really just being near people in general. This symptom is typically indicative of ringworm, a fungal infection that can be spread through direct contact with an infected person as well as through contact with objects a person with the condition has used, like towels, sheets, and household surfaces.

If you notice new blisters on your skin—especially ones that burst and crust over—then keep your hands to yourself. Impetigo, a skin infection most commonly caused by Staphylococcus and Streptococcus strains of bacteria, can be transmitted both by skin-to-skin contact and via contaminated items like clothing, bedding, and towels.

While many people think of scarlet fever as an antiquated malady, it's actually still a highly contagious disease spread by an all-too-common infection: strep throat. All someone with strep throat has to do is contaminate a surface or even cough near you, and that's enough to put you at risk.

If you've got shingles—a viral infection caused by herpes zoster—be careful about who you come in contact with. "There is virus in those blisters," says David Cutler, MD, family medicine physician at Providence Saint John's Health Center in Santa Monica, California.

And it's not just shingles that you can spread when you're infected with herpes zoster. Since the virus that causes shingles also causes chicken pox, "you're worried about coming in contact with people who are immunocompromised, on chemotherapy, or infants." The good news? According to the Centers for Disease Control and Prevention (CDC), the virus is only active when an affected individual has blisters—so once your itchy patches have crusted over, you're free to head outside again.

While it's not transmissible through traditional contact, there's evidence to suggest that depression can be spread among groups. "There is research proving that individuals suffering from major depressive disorder (MDD) affect those around them," says Dr. Nikola Djordjevic, MD, co-founder of LoudCloudHealth.com. "Those in close proximity of friends and family that suffer from depression can begin to develop symptoms of depression themselves. Whether it will 
escalate to MDD is unknown, but the fact [that] it affects others in their proximity is enough to categorize it as a contagious disease."

Though you can't get breast cancer from hugging someone with the disease, there is evidence to suggest that this condition is transmissible through organ donation. In fact, one 2018 case report published in the American Journal of Transplantation highlights how four individuals who received the kidneys, lungs, and liver of a single donor all developed breast cancer anywhere from six to 16 years after undergoing their respective organ transplants.

Breastfeeding may have its benefits, but it can also be the source of an unpleasant communicable condition: an oral yeast infection known as thrush.

"When a breastfeeding parent has candidiasis of the nipple, the corresponding diagnosis in the newborn is thrush," explains Ruiz. "Through the process of breastfeeding, the child and the mother pass the infection back and forth—you actually have to treat both." Fortunately, the remedy is relatively non-invasive: an oral swab for the baby and a nystatin cream for the parent.

In additional to oral yeast infections, you can also experience fungal overgrowth in your genital area. This can be the result of taking antibiotics or, Ruiz explains, "if you're exercising and staying in your exercise clothing too long."

Though yeast infections typically aren't contagious, they can be passed from one partner to another through sex, so you're better safe than sorry.

Anthrax—a potentially deadly infectious disease—is, in fact, communicable. But the odds you'll get it are incredibly slim since "it's an animal-borne disease," as Cutler explains.

So, if it's more common amongst animals, how can humans get anthrax, which can lead to respiratory failure, fever, flu-like symptoms, and even death? Contaminated food, water, and air can all be transmission sources. Also, getting anthrax spores in an open wound can cause a person to catch this frightening ailment.

While your chances of catching rabies from another person are slim, the possibility still exists. "Bite and non-bite exposures from an infected person could theoretically transmit rabies," cautions the CDC. Luckily, the overwhelming majority of rabies cases are transmitted through bites from an afflicted animal, so don't lose sleep at night worrying that your foaming friend might infect you.

If you're feeling lonely, don't be surprised if others around you start to feel the same. While not a biological contagion, loneliness can actually spread through social networks in a manner similar to traditional pathogens, according to a 2009 review of research published in the Journal of Personality and Social Psychology.

While obesity isn't contagious in a traditional sense, there is some evidence to suggest that it also spreads through social networks. Take, for example, this 2007 study published in the New England Journal of Medicine that followed a network of 12,067 people over a 23-year period. The researchers found distinct clusters of obesity among social networks, leading the study's authors to conclude that "obesity appears to spread through social ties." And if you're looking to shed those extra pounds, here are the 100 Ultimate Weight-Loss Tips.

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