Susceptibility to disease means that a person is likely to fall ill, if they are in contact with germs.
Many diseases kill people, so those that are more susceptible to diseases are more likely to die before their reproductive age. Some diseases make people infertile; so in short - susceptibility to disease means that people are less likely to reproduce and spread their genes, so the entire population will be less susceptible to these diseases in the future - and this process is called natural selection!
