This is an antiviral drug-mechanism module structured as a biological pathway, not a specific disease. Its nodes are the steps of the host-dependency logic (viral dependence on host factors -> engagement of a specific druggable host factor -> host-directed blockade -> broad-spectrum suppression of replication), with an escape branch (host-route rewiring / tropism shift and on-target host toxicity). The canonical conformance / treatment target is "Host Receptor and Protease Engagement" — the specific druggable host factor (e.g., ACE2/TMPRSS2 for SARS-CoV-2). A host-directed treatment (camostat/nafamostat against TMPRSS2, a soluble-receptor decoy, or a host-metabolic-pathway inhibitor) points its target_mechanisms edge at that node. The distinguishing feature from the direct-acting antiviral modules is encoded in the blockade and consequence nodes: because the target is a conserved host factor, one agent can act across viral families and the virus cannot trivially mutate the target — but the escape node records that the virus can instead switch to an alternative host factor/route, and that host-protein targeting carries intrinsic cytotoxicity risk. See projects/ANTIVIRAL.md and projects/RESPIRATORY_INFECTIONS.md.
Viral Dependence on Host Dependency Factors
trigger
Viruses are obligate intracellular parasites that cannot replicate autonomously: every stage of the life cycle — attachment, entry, genome replication, translation, assembly, and egress — co-opts host-cell proteins, membranes, and metabolic pathways collectively called host dependency factors. Cell entry is the archetype: coronaviruses depend on binding of the viral spike glycoprotein to a cellular receptor and on priming of that spike by a host-cell protease. This obligate reliance on the host is the premise of the entire module — it is what makes a host factor, rather than a viral enzyme, a viable antiviral target.
Downstream
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Host Receptor and Protease Engagement
The general dependence resolves into engagement of specific, identifiable host factors that can be named and drugged.
Host Receptor and Protease Engagement
effector
The general dependence on the host resolves into a small number of specific, druggable host factors. The canonical worked example is SARS-CoV-2, which uses the host receptor ACE2 for entry and the host serine protease TMPRSS2 to prime its spike protein for membrane fusion; a TMPRSS2 inhibitor already approved for clinical use blocks entry. This identifiability is what converts the abstract "viruses need the host" premise into a concrete drug target, and it is the canonical conformance/treatment target of the module: a host-directed agent — the TMPRSS2 inhibitors camostat and nafamostat, or a soluble ACE2 receptor decoy that competitively sequesters the spike — points at this node. Because the engaged factor (ACE2, TMPRSS2, or a shared metabolic enzyme) is host-encoded, the same node can be the target of drugs deployed against several different viruses.
Used by disorders
COVID-19
as SARS-CoV-2 Spike-Mediated Entry via ACE2 and TMPRSS2
Downstream
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Host-Directed Antiviral Blockade
Pharmacologically blocking the engaged host factor interrupts the viral life cycle without targeting any viral protein.
Host-Directed Antiviral Blockade
therapeutic vulnerability
Blocking a host dependency factor — rather than a viral enzyme — defines the host-directed antiviral strategy. Its decisive advantage follows directly from the target's biology: because the host factor is encoded by the comparatively stable host genome, the virus cannot escape by a simple point mutation in its own genome, so host-directed agents impose a higher genetic barrier to resistance than direct-acting antivirals. The same property makes a single agent potentially active against any virus that depends on that factor. This node is the therapeutic-rationale hinge of the module: the conformance/treatment edge sits one step upstream at the engaged host factor, but the reason to aim there is captured here.
Downstream
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Broad-Spectrum Suppression of Viral Replication
Engaging a shared host factor suppresses replication across multiple, unrelated viruses.
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Host-Factor Escape, Tropism Shift, and On-Target Toxicity
The strategy fails when the virus rewires to an alternative host route, or when targeting a host protein produces host-cell toxicity.
Broad-Spectrum Suppression of Viral Replication
consequence
The biological output of engaging a conserved host factor is suppression of viral replication that, unlike a virus-specific direct-acting antiviral, can extend across multiple unrelated viruses sharing that factor — the broad-spectrum promise that makes host-directed therapeutics attractive for pandemic preparedness, where the next threatening virus is not known in advance. This is the consequence node: it sits downstream of host-factor blockade and is the population-level antiviral effect a host-directed agent is deployed to achieve.
Host-Factor Escape, Tropism Shift, and On-Target Toxicity
adaptive escape
Host-directed antivirals are not escape-proof, and they carry a liability that direct-acting antivirals do not. First, a virus can evade the strategy not by mutating the drug target but by rewiring to an alternative host route: SARS-CoV-2 Omicron inefficiently uses the surface protease TMPRSS2 and instead relies more on the endosomal, cathepsin-dependent entry pathway, shifting its tropism and blunting the rationale for a TMPRSS2 inhibitor. Second, because the target is a host protein with its own physiological role, host-directed agents risk on-target cytotoxicity or disruption of normal cell function — a constraint that must be weighed in clinical development. This adaptive/limitation branch off the blockade node encodes why a host-directed agent's effect may erode (route switching) and why the therapeutic window is intrinsically constrained (host toxicity).