# Epidemics¶

During this seminar we will numerically solve systems of differential equations of SI, SIS and SIR models. <br> This experience is going to help us as we switch to network models.

## SI model¶

In this model a sustainable infection process is considered. Infected part of population has no chance to be healed..
In other words: $$\begin{cases} \cfrac{ds(t)}{dt} = -\beta s(t)i(t)\\ \cfrac{di(t)}{dt} = \beta s(t)i(t) \end{cases} \\ i(t) + s(t) = 1$$

In [1]:
import numpy as np
import matplotlib.pyplot as plt
from scipy.integrate import odeint
%matplotlib inline

In [8]:
# spreading coefficient
beta = 0.2

# initial state
i0 = 0.6
z0 = [1-i0, i0]

# time domain
t = np.arange(50)

# system of differential equations..
def si(z, t, beta):
return np.array([
-beta * z[1] * z[0],
beta * z[1] * z[0]])

# solved
z = odeint(si, z0, t, (beta,))

In [14]:
# Lets plot our solution and phase-plot
fig, ax = plt.subplots(1,2,figsize=(14,6))
lines = ax[0].plot(z)
plt.setp(lines[0], color='blue')
plt.setp(lines[1], color='red')
ax[0].set_xlabel('$t$')
ax[0].set_ylabel('proportion')
ax[0].legend(['$S$', '$I$'])
ax[1].plot(z[:,1], z[:,0])
ax[1].set_xlabel('$I$')
ax[1].set_ylabel('$S$')

Out[14]:
<matplotlib.text.Text at 0x7f51215e66d0>

The cool thing is that we can set $\beta$ and $\gamma$ to be dependent on $t$, that is interpreted as some ''sessional'' profile of the desease. <br> Now, based on this code, implement SIS and SIR models:

## SIS model¶

SIS model allowes infected agents to be cured, but without any further immunity. $$\begin{cases} \cfrac{ds(t)}{dt} = -\beta s(t)i(t) + \gamma i(t)\\ \cfrac{di(t)}{dt} = \beta s(t)i(t) - \gamma i(t) \end{cases} \\ i(t) + s(t) = 1$$ Implement this model and check cases when $\gamma \lessgtr \beta$

In [62]:
beta = 0.5
gamma = 0.1

# initial state
i0 = 0.6
z0 = [1-i0, i0]

# time domain
t = np.arange(50)

# system of differential equations..
def sis(z, t, beta, gamma):
return np.array([
-beta * z[1] * z[0] + gamma * z[1],
beta * z[1] * z[0] - gamma * z[1]])

# solved
z = odeint(sis, z0, t, (beta,gamma))

In [61]:
# Lets plot our solution and phase-plot
fig, ax = plt.subplots(1,2,figsize=(14,6))
lines = ax[0].plot(z)
plt.setp(lines[0], color='blue')
plt.setp(lines[1], color='red')
ax[0].set_xlabel('$t$')
ax[0].set_ylabel('proportion')
ax[0].legend(['$S$', '$I$'])
ax[1].plot(z[:,1], z[:,0])
ax[1].set_xlabel('$I$')
ax[1].set_ylabel('$S$')

Out[61]:
<matplotlib.text.Text at 0x7f511f595110>

## SIR model¶

In SIR model healed population gain immunity to the infection $$\begin{cases} \cfrac{ds(t)}{dt} = -\beta s(t)i(t)\\ \cfrac{di(t)}{dt} = \beta s(t)i(t) - \gamma i(t)\\ \cfrac{dr(t)}{dt} = \gamma i(t) \end{cases} \\ i(t) + s(t) + r(t) = 1$$

In [74]:
# put your code here
beta = 2
gamma = 0.6

# initial state
i0 = 0.2
r0 = 0
z0 = [1-i0-r0, i0, r0]

# time domain
t = np.arange(50)

# system of differential equations..
def sir(z, t, beta, gamma):
return np.array([
-beta * z[1] * z[0],
beta * z[1] * z[0] - gamma * z[1],
gamma * z[1]])

# solved
z = odeint(sir, z0, t, (beta,gamma))

In [75]:
# Lets plot our solution and phase-plot
fig, ax = plt.subplots(1,2,figsize=(14,6))
lines = ax[0].plot(z)
plt.setp(lines[0], color='blue')
plt.setp(lines[1], color='red')
plt.setp(lines[2], color='green')
ax[0].set_xlabel('$t$')
ax[0].set_ylabel('proportion')
ax[0].legend(['$S$', '$I$', '$R$'])
ax[1].plot(z[:,1], z[:,0])
ax[1].set_xlabel('$I$')
ax[1].set_ylabel('$S$')

Out[75]:
<matplotlib.text.Text at 0x7f511ea69350>
In [ ]: