diff --git a/avida-core/source/main/cDeme.cc b/avida-core/source/main/cDeme.cc
index d954cc63d5..6887c5b20e 100644
--- a/avida-core/source/main/cDeme.cc
+++ b/avida-core/source/main/cDeme.cc
@@ -350,6 +350,22 @@ int cDeme::GetNumParasites() const
   return count;
 }
 
+void cDeme::UpdateParasiteMemoryScore(const double decay) {
+  if (GetSize() == 0) return;
+  const double inc = static_cast<double>(GetNumParasites()) / GetSize();
+  SetParasiteMemoryScore(GetParasiteMemoryScore() * decay + inc);
+}
+
+void cDeme::SetParasiteMemoryScore(const double score) {
+  if (!std::isfinite(score)) {
+    cerr << "ERROR: SetParasiteMemoryScore() called with " << score
+      << "current score is " << GetParasiteMemoryScore()
+      << std::endl;
+    exit(1);
+  }
+  parasite_memory_score = score;
+}
+
 void cDeme::ProcessPreUpdate()
 {
   deme_resource_count.SetSpatialUpdate(m_world->GetStats().GetUpdate());
diff --git a/avida-core/source/main/cDeme.h b/avida-core/source/main/cDeme.h
index 7fbfb25ef2..9da95095e4 100644
--- a/avida-core/source/main/cDeme.h
+++ b/avida-core/source/main/cDeme.h
@@ -192,14 +192,9 @@ class cDeme
     else return static_cast<double>(GetNumParasites()) / GetOrgCount();
   }
 
-  void UpdateParasiteMemoryScore(const double decay) {
-    parasite_memory_score += static_cast<double>(GetNumParasites()) / GetSize();
-    parasite_memory_score *= decay;
-  }
+  void UpdateParasiteMemoryScore(const double decay);
   double GetParasiteMemoryScore() const { return parasite_memory_score; }
-  void SetParasiteMemoryScore(const double score) {
-    parasite_memory_score = score;
-  }
+  void SetParasiteMemoryScore(const double score);
 
   void IncOrgCount() { cur_org_count++; }
   void DecOrgCount() { cur_org_count--; }