diff --git a/cara/models.py b/cara/models.py
index a62e73d5..2b4b66b9 100644
--- a/cara/models.py
+++ b/cara/models.py
@@ -669,6 +669,14 @@ class _PopulationWithVirus(Population):
     #: The virus with which the population is infected.
     virus: Virus
 
+    @method_cache
+    def fraction_of_infectious_virus(self) -> _VectorisedFloat:
+        """
+        The fraction of infectious virus.
+
+        """
+        return 1.
+
     @method_cache
     def emission_rate_when_present(self) -> _VectorisedFloat:
         """
@@ -714,8 +722,18 @@ class InfectedPopulation(_PopulationWithVirus):
     expiration: _ExpirationBase
 
     #: The ratio of virions that are inactivated by the infected person's immunity.
+    # This parameter considers the potential antibodies in the infected person, 
+    # which might render inactive some RNA copies (virions). 
     host_immunity: _VectorisedFloat
 
+    @method_cache
+    def fraction_of_infectious_virus(self) -> _VectorisedFloat:
+        """
+        The fraction of infectious virus.
+
+        """
+        return self.virus.viable_to_RNA_ratio * (1 - self.host_immunity)
+
     @method_cache
     def emission_rate_when_present(self) -> _VectorisedFloat:
         """
@@ -969,11 +987,12 @@ class ExposureModel:
     def infection_probability(self) -> _VectorisedFloat:
         exposure = self.exposure()
 
+        f_inf = self.concentration_model.infected.fraction_of_infectious_virus()
+
         inf_aero = (
             self.exposed.activity.inhalation_rate *
             (1 - self.exposed.mask.inhale_efficiency()) *
-            exposure * self.fraction_deposited * 
-            (self.concentration_model.infected.virus.viable_to_RNA_ratio * (1 - self.concentration_model.infected.host_immunity))
+            exposure * self.fraction_deposited * f_inf
         )
         
         # Probability of infection.