Vehicle transmission, transmission by an inanimate reservoir (food, water, soil)
Vertical transmission, directly from the mother to an embryo, fetus or baby during pregnancy or childbirth. It can occur as a result of a pre-existing infection or one acquired during pregnancy. Iatrogenic transmission, due to medical procedures such as injection or transplantation of infected material. Vector-borne transmission, transmitted by a vector, which is an organism that does not cause disease itself but that transmits infection by conveying pathogens from one host to another.The relationship between virulence versus transmissibility is complex; with studies have shown that there were no clear relationship between the two. There is still a small number of evidence that partially suggests a link between virulence and transmissibility. Diagnosis
Diagnosis of infectious disease sometimes involves identifying an infectious agent either directly or indirectly. In practice most minor infectious diseases such as warts, cutaneous abscesses, respiratory system infections and diarrheal diseases are diagnosed by their clinical presentation and treated without knowledge of the specific causative agent. Conclusions about the cause of the disease are based upon the likelihood that a patient came in contact with a particular agent, the presence of a microbe in a community, and other epidemiological considerations. Given sufficient effort, all known infectious agents can be specifically identified. The benefits of identification, however, are often greatly outweighed by the cost, as often there is no specific treatment, the cause is obvious, or the outcome of an infection is benign. Diagnosis of infectious disease is nearly always initiated by medical history and physical examination.
Up to 30% of those infected, including 80% of the children under 5 years of age, died from the disease, and one-third of the survivors went blind. In the 19th century, tuberculosis killed an estimated one-quarter of the adult population of Europe; by 1918 one in six deaths in France were still caused by TB. The Influenza Pandemic of 1918 (or the Spanish flu) killed 25–50 million people (about 2% of world population of 1.7 billion). Today Influenza kills about 250,000 to 500,000 worldwide each year. Emerging diseases
In most cases, microorganisms live in harmony with their hosts via mutual or commensal interactions. Diseases can emerge when existing parasites become pathogenic or when new pathogenic parasites enter a new host. Coevolution between parasite and host can lead to hosts becoming resistant to the parasites or the parasites may evolve greater virulence, leading to immunopathological disease. Human activity is involved with many emerging infectious diseases, such as environmental change enabling a parasite to occupy new niches. When that happens, a pathogen that had been confined to a remote habitat has a wider distribution and possibly a new host organism. Parasites jumping from nonhuman to human hosts are known as zoonoses. Under disease invasion, when a parasite invades a new host species, it may become pathogenic in the new host.Several human activities have led to the emergence of zoonotic human pathogens, including viruses, bacteria, protozoa, and rickettsia, and spread of vector-borne diseases, see also globalization and disease and wildlife disease:

Encroachment on wildlife habitats.