a St oe
Resistance. We need to account for:

{a} The already existing resistance to infection.
(b} The resistance which develops during the infection.
(c) The resistance which persists after infection.

These questios can best be answered by a study of the phenomena which
are grouped under the title of immunity.

Immunity is the power which certain living organisms possess of
resisting infection. It is a specific pover.

Virulence or pathogenicity is the ability of a micro-organism to over-
come the” protective méchanisms of the host. The action of these organisms
and reaction of the host results in whet we call disease.

Susceptibility is due to the absence or suppression of the factors
which underlie immunity.

Types of Immunity:

{a) Naturel--mey be inherited by an individual or a whole species.
(pb) Aequired--through artificial means; may be either (a) active, or
(b) passive.
1. Actives
(1) Attack of disease
{2) Introduction of virus or live organisms

(3) Introduction of vaccine or dead organisms
(4) Introductiaom of toxins

2. Passive:
{1) Introduction of anti-bodies from another organism.

Theories of Immnity:

The science of immunology is only 30 years old. The explanation of
it has been a subject of discussion for centuries, ven primitive peoples
have tried to immunize themselves. There have been many theories.

(a) Exhaustion {Pasteur}
(b) Retention
(c) Ehrlichs-~Side-chain

Duration of Immunity

(a) Transient--in a great many infections: diphtheria (from attack of
disease}; tonsillitis; comnon colds; influenza.

{b) Permanent--appears to be for life of the individual: smallpox;
measles: typhoid fever; mumps; poliomyelitis; chicken-pox; etc.

Tests for Immunity

Scientists have developed a number of tests which will determine
whether an individual is susceptible to a specific disease.