Why are there not vaccines available for all diseases?

What is vaccine?

A vaccine is a substance introduced in to a person to stimulate the immunity against a particular infectious agent, thereby providing protection against the infection in the future.

A vaccine usually contains the dead organism or a non infectious modified (attenuated) form of the live organism. The body produces antibodies against specific proteins (antigen) on the infectious agent.

Bugs like bacteria or viruses are very smart. They learn to adapt to new environments to survive. They do this by changing the structure or types of antigen on their surface. Therefore, a vaccine effective against one antigen of the bacteria or virus will be ineffective against the same antigen if it undergoes modification.

The antigen change is the reason why effective vaccines have not been developed against HIV and every year a new flu vaccine has to be developed.

Vaccines are used to protect diseases and not infections.The breakthroughs during the last century in medical field is the development of vaccines to protect against viruses like:smallpox,chickenpox,HPV,polio,hepatitis and not so far against HIV.The main obstacle to find an effective vaccine on HIV,this virus mutates quickly.Most vaccines are tested on animals but in the case of HIV,no typical model of animals are available.Any testing that has been done on animals has not shown how humans would react to the tested vaccine.Any way the trial is going on in different countries like Thailand,Rwanda,South Africa,USA.

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