Answer:HIV attacks a specific type of immune system cell in the body. It's known as the CD4 helper cell or T cell. When HIV destroys this cell, it becomes harder for the body to fight off other infections. When HIV is left untreated, even a minor infection such as a cold can be much more severe.
Explanation:
 
        
             
        
        
        
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Answer:</h2>
<u>Blood clotting</u> is a positive feedback.
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Explanation:</h2>
All the physiological process occurring in the body are monitored by two types of mechanism called negative feedback and positive feedback.  
Positive feedback is the reaction which occurs with the feedback from the actions the system has performed. The blood clot occurs with palette formation which is regulated with the blood lose (continuous feedback).
Negative feedback is a reaction that is the invert of the change distinguished, meaning it capacities to decrease the change. A change is recognized by a receptor and an effector is made to incite a contrary impact which facilitates further reaction.  
The physiological process such as shivering, sweating and digestion are examples of negative feedback.
 
        
             
        
        
        
<span>In
the process of polymerization, a chain of similar smaller molecules
called monomer join together to create very large molecules called
polymer. Through this chemical reaction, water is created as a
by-product. In contrast to hydrolysis or breaking down of polymers, a 
water molecule is used to break the covalent bond that hold the
components of a polymer.</span>