Most people imagine infectious agents as viruses or bacteria, organisms with recognizable shapes, protein coats, and complex machinery. Beneath those familiar categories exists a stranger world, tiny circular RNA molecules so small and so simple that they challenge our definition of life. These are viroids and viroid-like agents, subviral RNA replicators that behave differently depending on the environment they inhabit.
Their story begins in 1971, when plant pathologist Theodor Diener discovered the first viroid while investigating a mysterious disease in potatoes. What he found was astonishing, an infectious agent made only of RNA, with no proteins, no capsid, and no viral machinery. It was smaller than any virus known at the time, yet capable of causing severe disease in plants. This discovery opened an entirely new chapter in plant pathology.
Viroids infect plants exclusively. They slip into plant cells and hijack the plant’s own RNA polymerase, tricking the cell into copying the viroid instead of its own genetic material. This interference disrupts normal gene expression and leads to stunted growth, twisted stems, yellowing leaves, malformed fruit, and dramatic yield loss. Potatoes, tomatoes, citrus trees, hops, avocados, and chrysanthemums are among the crops most heavily affected. In agriculture, viroids can be devastating, silently reducing yields and weakening entire orchards or fields.
Despite their destructive power in plants, viroids pose no threat to humans. They cannot enter human cells, cannot replicate in human tissues, and cannot interact with human RNA pathways. Their entire life cycle is locked to plant biology, and even heavily infected produce is harmless to people.
The story becomes more intriguing when we look at the human microbiome. In 2024, researchers analyzing saliva and stool samples discovered a new class of viroid-like RNA molecules they named obelisks. These tiny circular RNAs resemble viroids in shape and structure, yet they behave differently. Unlike true viroids, obelisks encode small proteins called Oblins, and they appear to replicate inside bacteria, particularly Streptococcus sanguinis, a common and beneficial oral microbe.
Streptococcus sanguinis normally lives in the mouth, where it helps maintain microbial balance and competes against cavity-causing bacteria. Every swallow of saliva carries millions of oral microbes into the digestive tract. Under healthy conditions, stomach acid, digestive enzymes, and a thriving gut microbiome keep these visitors in check. Not all bacteria are killed, and some survive the journey, but most are quickly outcompeted by native gut species.
When the digestive environment shifts because of low stomach acid, stress, antibiotics, or depleted gut flora, oral bacteria can survive longer than usual. If they reach the intestines in high enough numbers, they can disrupt the balance of beneficial gut microbes. Oral bacteria ferment carbohydrates differently, compete for nutrients, and can crowd out species that produce butyrate, a compound essential for maintaining the integrity of the gut lining. When butyrate-producing bacteria are weakened, the gut barrier can become more permeable, allowing inflammatory molecules to pass more easily into circulation.
This does not cause infection or disease, but it does create imbalance, a subtle shift in the microbial landscape that affects how the gut lining is nourished and protected.
When oral bacteria migrate, their microbial passengers migrate with them. Obelisks, the tiny circular RNAs found in Streptococcus sanguinis, travel wherever their bacterial hosts travel. They do not infect human cells or cause illness, yet their presence reveals how interconnected microbial ecosystems truly are. These RNA elements behave differently depending on the environment, destructive in plants, harmless in human-associated bacteria, and possibly ancient remnants of an early RNA-based world.
Together, viroids and obelisks show us that biology is not always grand or visible. Sometimes it is small, silent, and astonishingly simple. These tiny RNA circles remind us that ecosystems, whether in soil, leaves, saliva, or the gut, are shaped by travelers we cannot see, yet whose presence influences the balance of life. Sometimes the smallest molecules tell the biggest stories.