This is the final element in the Court’s analysis, although it is more a synthesis of the first three elements. The probability of transmission depends on the mode of transmission and the infectivity of the diseases. This is sometimes called contact effectiveness. The higher the contact effectiveness is, the higher is the probability that a person coming into proper contact with a disease carrier will contract the disease. For example, HIV is a disease with a low contact effectiveness; a sexual encounter has a less than 5% chance of spreading the disease. There is no possibility of spreading HIV by casual contact such as coughing. Conversely, measles is a disease with a high contact effectiveness. A single exposure to a coughing person infected with measles usually results in infection among susceptible persons.

If the usual mode of transmission does not occur in the workplace, then there is a very low probability of transmission. This is the usual case for sexually transmitted diseases that are not otherwise bloodborne. If the mode of transmission occurs in the workplace but the contact efficiency is very low, there is also a low probability of transmission. This would be the case for a disease such as leprosy that is spread by physical contact but the contact efficiency is so low that it poses a risk only to intimate family members. Conversely, active tuberculosis, an airborne disease with a medium contact efficiency, poses a threat to anyone with whom the infected person has frequent contact.

The Court did not specifically address the problem of persons with special susceptibility to communicable diseases—primarily immunosuppressed persons and pregnant women. The court has ruled that fertile women as a class cannot be excluded from workplaces where there is exposure to teratogens. It is likely that current law prevents pregnant women from being excluded from workplaces where there is only a possibility of infection with a disease that adversely affects the fetus. If there is a high probability of exposure, as on a hospital service caring for persons infected with the disease, the court might allow some restrictions on pregnant workers. The limited set of infections that pose a risk to the fetus and the fixed length of pregnancy would make pregnancy- related restrictions unusual outside certain medical care workplaces.