This varies greatly, some animals readily succumbing to infection by a certain organism, while other animals will be distinctly resistant. This resistance, however, may depend very largely upon the physical condition of the invaded individual. If anything occurs that will lower the general physiologic activity, the individual will then be less able to withstand the attack of the organisms. Fatigue is an apparent factor. When tired out, from one cause or another, infection is much more likely to occur. Exposure, particularly to cold, is a very common cause of lowering the bodily resistance, and thus allowing infection. Diet appears to have some obscure effect in predisposing to certain diseased conditions. Intoxication by poisonous substances increases the susceptibility to infection. This is quite commonly seen in the frequency and fatality of pneumonia among excessive indulgers in alcohol. Injuries of all sorts render the possibility of infection to become greater.

When two different types of organisms invade the body at the same time the resulting condition is known as a mixed infection.

If after one organism has caused tissue changes, another gains entrance and gives rise to pathologic conditions, it is called a secondary infection.