However, this does not mean that small populations never have any contagious disease. They are also infected, but only infected with certain diseases. Cats She Lived Happily Ever After poster. Some are caused by germs that are able to live in animals or in the soil, resulting in the disease not disappearing but still often able to infect humans. For example, the typhus virus is carried in wild African monkeys, so it can always infect rural populations in Africa, from which they are carried with ships. transatlantic slave trade that infects monkeys and humans in the New World.