Microbes are a practical category, not a strict biological one
The word "microbe" is not a formal taxonomic rank like "mammal" or "plant." It is a working category for organisms and agents that are too small to see without a microscope and are studied with similar tools. A book on viral ecology notes that efforts to define a virus inevitably raise the question of what constitutes a microbe, which suggests the boundary is debated in practice [3].
In that practical sense, viruses fit comfortably. They are tiny, they are everywhere in the same samples as bacteria, and researchers routinely analyze them together. A study of bat-borne viruses describes bats as a reservoir of emerging zoonotic microbes, including viruses, and then classifies those viruses using the same international taxonomy system used for other infectious agents [1].
Viruses live in the same places and depend on the same hosts
Viruses are found in the same environments as microbes and are often detected through the same sampling methods. In a study of estuarine water, researchers used a molecular technique to link viral and microbial DNA from the same cells, identifying 18 virus-microbe interactions, including viruses associated with bacterial groups that had no previously known viruses [4].
In the rumen, a study analyzed 551 metagenomes and found that viruses may regulate microbes at both strain and community levels through antagonistic and mutualistic interactions [2]. The viruses are not separate from the microbial community; they are part of how that community behaves and diversifies.
Viruses are counted and classified as part of microbial communities
When researchers survey a habitat, they often count viruses alongside bacteria and other microbes. In Oklahoma soils, virus-like particle abundance varied across sites and seasons and correlated with environmental factors such as air temperature and soil nitrogen, and the authors describe viruses as influencing microbial abundance, community structure, and evolution [5].
Classification also pulls viruses into the microbial fold. The bat virus study identified 120 putative viral species using the International Committee on Taxonomy of Viruses criteria, treating viruses as classifiable biological entities with evolutionary lineages [1]. That does not make viruses cells, but it does make them part of the same scientific inventory as microbes.
The label is about how viruses are studied, not what they are made of
Calling viruses microbes is a statement about scale and ecological role, not about cellular structure. Viruses lack cells and cannot reproduce on their own, which is why their status as living organisms is debated. The supplied evidence does not settle that debate; it shows that in ecology and taxonomy, viruses are handled as part of the microbial world because they are microscopic, abundant, and inseparable from the microbes they infect [2][4][5].
So the practical answer is that viruses are considered microbes because that is the most useful category for studying them. They are sampled with microbes, counted with microbes, classified with microbes, and analyzed for how they interact with microbial hosts. The label reflects research practice more than a final verdict on what viruses fundamentally are [3].
Sources used in this answer
Unveiling bat-borne viruses: a comprehensive classification and analysis of virome evolution
The study classified bat-borne viruses using ICTV criteria, identifying 120 putative viral species and treating viruses as classifiable agents within a microbial reservoir.
Viruses contribute to microbial diversification in the rumen ecosystem and are associated with certain animal production traits
Analysis of 551 rumen metagenomes suggests rumen viruses may regulate microbes through antagonistic and mutualistic interactions and are associated with animal production traits.
Quantitative viral ecology: dynamics of viruses and their microbial hosts
The book notes that efforts to define a virus inevitably raise the question of what constitutes a microbe, indicating the boundary is debated in practice.
Hi-C assembled genomes of estuarine populations reveal virus-microbe associations and a broad interaction range of a cyanophage
Using Hi-C to link viral and microbial DNA in an estuary, the study identified 18 virus-microbe interactions, including viruses associated with previously unknown hosts.
Temporal Changes of Virus-Like Particle Abundance and Metagenomic Comparison of Viral Communities in Cropland and Prairie Soils
In Oklahoma soils, virus-like particle abundance varied seasonally and with land use, and viruses are described as influencing microbial abundance, community structure, and evolution.