Poisson Distribution 

% Clears screen and deletes all the variables in the workspace
clc; clear; 

% Asks the user for input
f = input('Calculated Frequency: ');
x = input('Test Frequency: '); 

% Computes probability
a = log(factorial(x));
a = exp(-f + x*log(f) - a); 

% Displays result
str = ['Probability of ' num2str(x) ' occurrences = ' num2str(a)];
disp(str)

The instruction 'num2str' (number-to-string) transforms a number into a string, to be used along with the rest of the output message, launched by instruction 'disp'.

 Example  1:

Some programs are downloaded into 2000 computers. The probability of any one computer getting infected by a virus is 0.001 (nothing to do with reality, ok?) Thus we can expect 2 computers will suffer a bad reaction (0.001 x 2000 = 2). What is the probability that 4 computers will get infected?

We launch our code above and enter known data as input:

Calculated Frequency: 2
Test Frequency: 4

Matlab answer is

Probability of 4 occurrences = 0.090224

Example 2:

What is the probability that only one computer will get infected?

We re-launch our code and enter data:

Calculated Frequency: 2
Test Frequency: 1