Such capsule formation is a very common thing in the pathogenic organisms, and they are found only when these are growing in the body and do not appear in cultures (Fig.

17-c).
It is evident that just as the parasite has his weapons of offence and defence so has the host, otherwise there would be no recovery from infectious diseases.

Although many of the infectious diseases have a high mortality, which in rare instances reaches one hundred per cent, the majority do recover.

In certain cases the recovery is attended by immunity, the individual being protected to a greater or less degree from a recurrence of the same disease.

The immunity is never absolute; it may last for a number of years only, and usually, if the disease be again acquired, the second attack is milder than the primary.