Definition of basic terms: Communicable disease: An illness due to specific infectious agent or its toxic products which develops through transmission of that agent or its toxic products from a reservoir to a susceptible host either directly or indirectly. Contagious disease - capable of spreading from one person to another Incubation period: The interval between exposure and first clinical manifestation. The time between exposure and first case is the minimum incubation period, The time between exposure and the last case, maximum incubation period.

Learning Objectives for the Lecture:
At the end of the lecture the student is going to be able to:
1. Define communicable and non-communicable diseases.
2. Describe the chain of infection.
3. Describe the links in the chain of infection.
Communicable Diseases
Communicable Diseases: They are diseases that spread by direct contact from infected person to a susceptible host.
Chain of infection: Series of events or conditions that lead to the development of communicable diseases.

Learning Objectives for the Lecture:
At the end of the lecture the student is going to be able to:
1. Describe the natural history of heath conditions.
2. Describe the stages of the natural history of heath conditions.
3. Describe the process of prevention.
4. Describe the levels of prevention.
Natural History of Health-related Conditions:
The natural history of health condition is a description of the events that precedes the development of the health condition, and during its occurrence through the course of development, as well as its outcome.

Learning Objectives for the Lecture:
At the end of the lecture the student is going to be able to:
1. Define epidemiology and related concepts.
2. Describe the purpose of epidemiology.
3. List the uses of epidemiology.
4. Identify the term of causality and its related terms.
5. Describe the epidemiological triad and the variations among its elements.
Simple Old Definitions
1. Study the occurrence of illnesses.
2. Branch of medical science which treats epidemics (Oxford English Dictionary).

In the preceding sections we studied parabolas with vertices at the origin and ellipses and
hyperbolas with centers at the origin. We restricted ourselves to these cases because these
equations have the simplest form. In this section we consider conics whose vertices and
centers are not necessarily at the origin, and we determine how this affects their equation 
 

Thyristor is, a three terminal, four layers solid state semiconductor device, each layer consisting of alternately N-type or P-type material, i.e; P-N-P-N.
The conventional thyristor is also known by the name SCR (Silicon Controlled Rectifier).
 They are operated as bistable switches, operating from non conducting state to conducting state.
Thyristors can be assumed as ideal switches for many application, but the practical thyristors exhibit certain characteristics and limitations. 
 

End as a subscript   
To access the last element of a matrix along a given dimension, use  
end as a subscript. This allows you to go to the final element without  
knowing in advance how big the matrix is. For example:  

>> q = 4:10    
q=   
4   5   6   7   8   9   10    

>> q(end)   
ans = 10  

 
>> q(end-4:end)    
ans = 6   7   8   9   10  

 
>> q(end-2:end)   
ans = 8   9   10  

The Colon Operator 
The colon" : " is one of MATLAB’s most important operators. It occurs 
in several different forms. The expression 
>>1:10 
is a row vector containing the integers from 1 to 10 
1 2 3 4 5 6 7 8 9 10 
To obtain nonunit spacing, specify an increment. For example, 
>>100:-7:50 
is 
100 93 86 79 72 65 58 51 
• End as a subscript 
To access the last element of a matrix along a given dimension, use end 
as a subscript. This allows you to go to the final element without knowing