The ability on the part of infectious agents to induce, incite, or produce pathogenic changes in the host is known as virulence. As Topley suggests, "virulent" titerally means "poisonous". It refers not only to invasiveness but also to changes caused by toxins elaborated by organisms. The hemolytic streptococcus is an excellent example ot an invader, while the diphtheria bacillus is a deadly toxin producer with feeble invasive power.

There is marked variation in the ability of different microorganisms, and among strains of the same microorganism, to invade the human body or to produce toxins poisonous to it. These differences are conditioned by many factors. The recent habitat of a parasite influences its virulence to a great extent. If a parasite has been freshly isolated from an animal body, it is likely to exhibit greater virulence than if it has been growing for some time on artificial media or kept in an environment which may be unfavorable to it, such as extreme heat or cold.

Infectious agents differ in their ability to withstand changes in the environment. Some, like the tubercle bacillus, have a waxy covering, which makes them difficult to destroy. Others, like the tetanus bacillus, preserve themselves under conditions unfavorable for growth by producing spores, vegetative forms which survive for long periods of time in a relatively inactive or resting state. When conditions for growth again become suitable, the spores change to the common bacterial form. Still others are quite readily adaptable to changes in the environment which lead to life remote from the host; the typhoid bacillus excreted in feces and urine is able to adapt itself to rather extreme changes and is capable of existence in or upon the soil or in water for relatively long periods of time.

Although many microorganisms are able to protect themselves against destruction, others are readily affected by sunlight, drying, and sudden changes or extremes of temperature. Meningococci soon die after removal from the human body and are relatively difficult to grow under optimum conditions on artificial media. In general, it can be stated thạt the more highly specialized an infectious agent becomes, that is, the more restricted its habitat and conditions for growth, the more readily it is destroyed when removed from its specific host.

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