diff --git a/ringvax/__init__.py b/ringvax/__init__.py
index 31cb4bf..205a4a0 100644
--- a/ringvax/__init__.py
+++ b/ringvax/__init__.py
@@ -19,10 +19,11 @@ class Simulation:
         "infection_times",
     }
 
-    def __init__(self, params: dict[str, Any], seed: Optional[int] = None):
+    def __init__(
+        self, params: dict[str, Any], rng: Optional[numpy.random.Generator] = None
+    ):
         self.params = params
-        self.seed = seed
-        self.rng = numpy.random.default_rng(self.seed)
+        self.rng = rng if rng is not None else numpy.random.default_rng()
         self.infections = {}
 
     def create_person(self) -> str:
diff --git a/ringvax/app.py b/ringvax/app.py
index cf9282e..c824d24 100644
--- a/ringvax/app.py
+++ b/ringvax/app.py
@@ -3,6 +3,7 @@
 
 import altair as alt
 import graphviz
+import numpy.random
 import polars as pl
 import streamlit as st
 
@@ -171,8 +172,12 @@ def app():
     sims = []
     with st.spinner("Running simulation..."):
         tic = time.perf_counter()
+
+        # initialize rngs
+        rngs = numpy.random.default_rng(seed).spawn(nsim)
+
         for i in range(nsim):
-            sims.append(Simulation(params=params, seed=seed + i))
+            sims.append(Simulation(params=params, rng=rngs[i]))
             sims[-1].run()
         toc = time.perf_counter()
 
diff --git a/tests/test_simulate.py b/tests/test_simulate.py
index 19ea7d5..2e89425 100644
--- a/tests/test_simulate.py
+++ b/tests/test_simulate.py
@@ -54,7 +54,7 @@ def test_generate_disease_history(rng):
         "infectious_duration": 1.0,
         "infection_rate": 2.0,
     }
-    s = ringvax.Simulation(params=params, seed=rng)
+    s = ringvax.Simulation(params=params, rng=rng)
     history = s.generate_disease_history(t_exposed=0.0)
     # for ease of testing, make this a list of rounded numbers
 
@@ -74,7 +74,7 @@ def test_generate_disease_history_nonzero(rng):
         "infectious_duration": 1.0,
         "infection_rate": 2.0,
     }
-    s = ringvax.Simulation(params=params, seed=rng)
+    s = ringvax.Simulation(params=params, rng=rng)
     history = s.generate_disease_history(t_exposed=10.0)
     assert history == {
         "t_exposed": 10.0,
@@ -100,7 +100,7 @@ def base_params():
 
 
 def test_simulate(rng, base_params):
-    s = ringvax.Simulation(params=base_params, seed=rng)
+    s = ringvax.Simulation(params=base_params, rng=rng)
     s.run()
     assert len(s.infections) == 19
 
@@ -108,20 +108,20 @@ def test_simulate(rng, base_params):
 def test_simulate_max_infections(rng, base_params):
     params = base_params
     params["max_infections"] = 10
-    s = ringvax.Simulation(params=params, seed=rng)
+    s = ringvax.Simulation(params=params, rng=rng)
     s.run()
     assert len(s.infections) == 10
 
 
 def test_simulate_set_field(rng, base_params):
-    s = ringvax.Simulation(params=base_params, seed=rng)
+    s = ringvax.Simulation(params=base_params, rng=rng)
     id = s.create_person()
     s.update_person(id, {"generation": 0})
     assert s.get_person_property(id, "generation") == 0
 
 
 def test_simulate_error_on_bad_get_property(rng, base_params):
-    s = ringvax.Simulation(params=base_params, seed=rng)
+    s = ringvax.Simulation(params=base_params, rng=rng)
     id = s.create_person()
 
     with pytest.raises(RuntimeError, match="foo"):
@@ -129,7 +129,7 @@ def test_simulate_error_on_bad_get_property(rng, base_params):
 
 
 def test_simulate_error_on_bad_update_property(rng, base_params):
-    s = ringvax.Simulation(params=base_params, seed=rng)
+    s = ringvax.Simulation(params=base_params, rng=rng)
     id = s.create_person()
 
     with pytest.raises(RuntimeError, match="foo"):